535 

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c  op  •  «j       J.~yTicLs.  JL1" 


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ELECTRICITY 


PROFESSOR  JOHN  TYNDALL'S  WORKS. 


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assembled  at  Belfast.  Revised,  with  Additions, 
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LESSONS 


ELECTRICITY 


AT  THE  EOYAL  INSTITUTION 


JOHN  TYNDALL,  D.O.L.,  LL.1X,  F.K.a 

PROFESSOR  OF  SATTJHAL  rnn^soFHT 

DJ  THE  ROYAL  DtSTTnrnON  OF  CKKAT  BTUTATO 


NEW  YORK: 
D.  APPLETON  AND  COMPANY, 

72    FIFTH    AVENUE. 
1895. 


. — A  Price-List  of  the  Apparatus  suitable  for  the 
experiments  described  in  these  Lessons  will  be  found  at  the  end 
of  the  volume.  The  teacher  or  learner  may  materially  reduce 
the  cost  by  becoming  his  own  instrument-maker. 


Q  C 


I  wish  to  inscribe  this  look  to  Five  Young  Fri&nds,  ivhose 
names,  in  the  approximate  order  of  their  ages,*  ire  here 
set  doivn  : — 

HUGH  SPOTTISWOODE. 

HENRY  HUXLEY, 

ROLFE  LUBBOCK, 

JOHN  CLAUSIUS, 

REGINALD  HOOKER. 


J.  T. 


•  I  write  in  Switzerland,  and  have  to  rely  upon  my  memory, 
hence  my  uncertainty 


PBEFACE. 


MORE  than  fifty  years  ago  the  Board  of  Managers  of 
the  Koyal  Institution  resolved  to  extend  its  usefulness, 
as  a  centre  of  scientific  instruction,  by  giving,  during 
the  Christmas  and  Easter  holidays  of  each  year,  two 
courses  of  Lectures  suited  to  the  intelligence  of  boys 
and  girls. 

On  December  12,  1825,  a  Committee  appointed  by 
the  Managers  reported  'that  they  had  consulted  Mr. 
Faraday  on  the  subject  of  engaging  him  to  take  a  part 
in  the  juvenile  lectures  proposed  to  be  given  during 
the  Christmas  and  Easter  recesses,  and  they  found  his 
occupations  were  such  that  it  would  be  exceedingly  in- 
convenient for  him  to  engage  in  such  lectures.' 

Faraday's  holding  aloof  was,  however,  but  tempo- 
rary, for  at  Christmas  1827  we  find  him  giving  a 
*  Course  of  Six  Elementary  Lectures  on  Chemistry, 
adapted  to  a  Juvenile  Auditory.' 

The  Easter  lectures  were  soon  abandoned,  but  from 
the  date  mentioned  to  the  present  time  the  Christmas 


viii  Preface. 

lectures  have  been  a  marked  feature  of  the  Koyal 
Institution.1 

Last  Christmas  it  fell  to  my  lot  to  give  one  of  these 
courses.  I  had  heard  doubts  expressed  as  to  the  value 
of  Science-teaching  in  schools,  and  I  had  heard  objec- 
tions urged  on  the  score  of  the  expensiveness  of  appa- 
ratus. Both  doubts  and  objections  would,  I  considered, 
be  most  practically  met  by  showing  what  could  be  done, 
in  the  way  of  discipline  and  instruction,  by  experi- 
mental lessons  involving  the  use  of  apparatus  so  simple 
and  inexpensive  as  to  be  within  everybody's  reach. 

With  some  amplification,  the  substance  of  our 
Christmas  Lessons  is  given  in  the  present  little  volume. 


1  These  brief  historic  references  have  already  appeared  in  the  Pre- 
face to  the  '  Forms  of  Water.' 


CONTENTS. 


WCT.  PAGB 

1.  INTRODUCTION      .           .  .  •           .           .1 

2.  HISTORIC  NOTES         .           .  .  .           .     .      1 

3.  THE  ART  op  EXPERIMENT  .  .           .           .4 

4.  MATERIALS  FOR  EXPERIMENT  .  .           .     .      5 
6.  ELECTRIC  ATTRACTIONS  .  .           .           .           .8 

6.  DISCOVERT  OF  CONDUCTION  AND  INSULATION          .     .13 

7.  THE  ELECTROSCOPE.    FURTHER  ENQUIRIES  ON  CONDUC- 

TION AND  INSULATION  .  .-          .  .15 

8.  ELECTRICS  AND  NON-ELECTRICS  .  .  .  .19 

9.  ELECTRIC   REPULSIONS.     DISCOVERT    OF   Two    ELEC- 

TRICITIES   .  .  .  .  .  .    22 

10.  FUNDAMENTAL  LAW  OF  ELECTRIC  ACTION        .  .    23 

11.  ELECTRICITT  OF  THE  RUBBER.     DOUBLE  OR  'POLAR' 

CHARACTER  OF  THE  ELECTRIC  FORCE       .  .     .    29 

12.  WHAT  is  ELECTRICITY?  .  .  .  .33 

13.  ELECTRIC  INDUCTION.     DEFINITION  OF  THE  TERM       .    3G 

14.  EXPERIMENTAL  RESEARCHES  ON  ELECTRIC  INDUCTION  .    38 


x  Contents. 

SECT.  PAGB 

16.    THE  ELECTROPHORUS  .  .  .  .     .    48 

16.  ACTION  OF  POINTS  AND  FLAMES  .  .  .51 

17,  THE  ELECTRICAL  MACHINE  .  .  .     .    65 

18  FURTHER  EXPERIMENTS  ox  THE  ACTION  OP  POINTS. 
THE  ELECTRIC  MILL.  THE  GOLDEN  FISH.  LIGHT- 
NING CONDUCTORS  .  .  .  .  .58 

19.  HISTORY  OF  THE  LEYDEN  JAR.    THE  LEYDEX  BATTERY    64 

20.  EXPLANATION  OF  THE  LEYDEN  JAR       .  .           .09 

21.  FRANKLIN'S  CASCADE  BATTERY        .  .        ...    72 

22.  NOVEL  LEYDEN  JARS  OF  THE  SIMPLEST  FORM  .    73 

23.  SEAT  OF  CHARGE  IN  THE  LEYDEN  JAR      .  .    77 

24.  IGNITION    BY    THE    ELECTRIC    SPARK.       COTTRELL'S 

RUBBER.     THE  TUBE -MACHINE  .  .  .     .    80 

25.  DURATION  OF  THE  ELECTRIC  SPARK      .  .  .85 
20.    ELECTRIC  LIGHT  IN  VACUO              .           .           .     .    88 

27.  LICHTENBERG'S  FIGURES  .  .  .  .94 

28.  SURFACE  COMPARED  WITH  MASS.      DISTRIBUTION  OF 

ELECTRICITY  IN  HOLLOW  CONDUCTORS     .  .     .    95 

29.  PHYSIOLOGICAL  EFFECTS  OF  THE  ELECTRIC  DISCHARGE    97 

30.  ATMOSPHERIC  ELECTRICITY         .  .  .  .99 
81.    THE  RETURNING  STROKE      .           .           .           .     .  102 

32.    THE  LEYIEN  BATTERY,  ITS  CURRENTS,  AND  SOME  OF 

THEIR  EFFECTS      .  .  .  .  .     .  108 

CONCLUSION  .111 


LESSONS   IN   ELECTEICITY, 


§  1.  Introduction. 

MANY  centuries  before  Christ,  it  had  been  observed 
that  yellow  amber  (elektron\  when  rubbed,  pos- 
sessed the  power  of  attracting  light  bodies. 

Thales,  the  founder  of  the  Ionic  philosophy  (B.C.  580), 
imagined  the  amber  to  be  endowed  with  a  kind  of  life. 

This  is  the  germ  out  of  which  has  grown  the  science 
of  electricity,  a  name  derived  from  the  substance  in 
which  this  power  of  attraction  was  first  observed. 

It  will  be  my  aim,  during  six  hours  of  these  Christ- 
mas holidays,  to  make  you,  to  some  extent,  acquainted 
with  the  history,  facts,  and  principles,  of  this  science, 
and  to  teach  you  how  to  work  at  it. 

The  science  has  two  great  divisions ;  the  one  called 
'  Frictional  Electricity,'  the  other  '  Voltaic  Electricity.' 
For  the  present,  our  studies  will  be  confined  to  the 
first,  or  older  portion  of  the  science,  which  is  called 
'  Frictional  Electricity,'  because  in  it  the  electrical  power 
is  obtained  from  the  rubbing  of  bodies  together. 

§  2.  Historic  Notes. 

The  attraction  of  light  bodies  by  rubbed  amber 
was  the  sum  of  the  world's  knowledge  of  electricity 


2  Lessons  in  Electricity. 

for  more  than  2,000  years.  In  1600  Dr.  Gilbert, 
physician  to  Queen  Elizabeth,  whose  attention  had 
been  previously  directed  with  great  success  to  mag- 
netism, vastly  expanded  the  domain  of  electricity.  He 
showed  that  not  only  amber,  but  various  spars,  gems, 
fossils,  stones,  glasses  and  resins,  exhibited,  when  rubbed, 
the  same  power  as  amber. 

Eobert  Boyle  (1675)  proved  that  a  suspended  piece 
of  rubbed  amber,  which  attracted  other  bodies  to  itself, 
was  in  turn  attracted  by  a  body  brought  near  it.  He 
also  observed  the  light  of  electricity,  a  diamond,  with 
which  he  experimented,  being  found  to  emit  light  when 
rubbed  in  the  dark. 

Boyle  imagined  that  the  electrified  body  threw  out 
an  invisible,  glutinous  substance,  which  laid  hold  of 
light  bodies  and,  returning  to  the  source  from  which 
it  emanated,  carried  them  along  with  it. 

Otto  von  Guericke.  Burgomaster  of  Magdeburg, 
contemporary  of  Boyle,  and  inventor  of  the  air-pump, 
intensified  the  electric  power  previously  obtained.  He 
devised  what  may  be  called  the  first  electrical  machine, 
which  was  a  ball  of  sulphur,  about  the  size  of  a  child's 
head.  Turned  by  a  handle,  and  rubbed  by  the  dry 
hand,  the  sulphur  sphere  emitted  light  in  the  dark. 

Von  Guericke  also  noticed,  and  this  is  important, 
that  a  feather,  having  been  first  attracted  to  his 
sulphur  globe,  was  afterwards  repelled,  and  kept  at 
a  distance  from  it,  until,  having  touched  another 
body,  it  was  again  attracted.  He  heard  the  hissing 
of  the  '  electric  fire,'  and  also  observed  that  an  un- 
electrified  body,  when  brought  near  his  excited  sphere, 
became  electrical  and  capable  of  being  attracted. 

The  members  of  the  Academy  del  Oimento  examined 


Historic  Notes.  3 

various  substances  electrically.  They  proved  smoke 
to  be  attracted,  but  not  flame,  which,  they  found, 
deprived  an  electrified  body  of  its  power 

They  also  proved  liquids  to  be  sensible  to  the 
electric  attraction,  showing  that  when  rubbed  amber 
was  held  over  the  surface  of  a  liquid,  a  little  eminence 
was  formed,  from  which  the  liquid  was  finally  dis- 
charged against  the  amber. 

Sir  Isaac  Newton,  by  rubbing  a  flat  glass,  caused 
light  bodies  to  jump  between  it  and  a  table.  He  also 
noticed  the  influence  of  the  rubber  in  electric  excita- 
tion. His  gown,  for  example,  was  found  to  be  much 
more  effective  than  a  napkin. 

Newton  imagined  that  the  excited  body  emitted  an 
elastic  fluid  which  penetrated  glass. 

In  the  efforts  of  Thales,  Boyle,  and  Newton  to  form 
a  mental  picture  of  electricity  we  have  an  illustration 
of  the  tendency  of  the  human  mind,  not  to  rest  satis- 
fied with  the  facts  of  observation,  but  to  pass  beyond 
the  facts  to  their  invisible  causes. 

Dr.  Wall  (1708)  experimented  with  large,  elongated 
pieces  of  amber.  He  found  wool  to  be  the  best  rubber 
of  amber.  *  A  prodigious  number  of  little  cracklings ' 
was  produced  by  the  friction,  every  one  of  them  being 
accompanied  by  a  flash  of  light.  '  This  light  and 
crackling,'  says  Dr.  Wall,  '  seem  in  some  degree  to  re- 
present thunder  and  lightning.' l  This  is  the  first 
published  allusion  to  thunder  and  lightning  in  connec- 
tion with  electricity. 

Stephen  Gray  (1729)  also  observed  the  electric 
brush,  snappings,  and  sparks.  He  made  the  prophetic 

1  'Phil.  Trans.'  ) 708,  p.  69. 


4  Lessons  in  Electricity. 

reuaaik  that  'though  these  effects  are  at  present  only 
minute,  it  is  probable  that  in  time  there  may  be  found 
out  a  way  to  collect  a  greater  quantity  of  the  electric 
fire,  and,  consequently,  to  increase  the  force  of  that 
power  which  by  several  of  those  experiments,  if  we  are 
permitted  to  compare  great  things  with  small,  seems 
to  be  of  the  same  nature  with  that  of  thunder  and 
lightning.' l  This,  you  will  observe,  is  far  more  definite 
than  the  remark  of  Dr.  Wall. 

§  3.  The  Art  of  Experiment. 

We  have  thus  broken  ground  with  a  few  historic 
notes,  intended  to  show  the  gradual  growth  of  electrical 
science.  Our  next  step  must  be  to  get  some  knowledge 
of  the  facts  referred  to,  and  to  learn  how  they  may  be 
produced  and  extended.  The  art  of  producing  and  ex- 
tending such  facts,  and  of  enquiring  into  them  by 
proper  instruments,  is  the  art  of  experiment.  It  is  an 
art  of  extreme  importance,  for  by  its  means  we  can,  as 
it  were,  converse  with  Nature,  asking  her  questions 
and  receiving  from  her  replies. 

It  was  the  neglect  of  experiment,  and  of  the  reason- 
ing based  upon  it,  which  kept  the  knowledge  of  the 
ancient  world  confined  to  the  single  fact  of  attraction 
by  amber  for  more  than  2,000  years. 

Skill  in  the  art  of  experimenting  does  not  come  of 
itself ;  it  is  only  to  be  acquired  by  labour.  When  you 
first  take  a  billiard  cue  in  your  hand,  your  strokes  are 
awkward  and  ill-directed.  When  you  learn  to  dance, 
your  first  movements  are  neither  graceful  nor  pleasant. 
By  practice  alone,  you  learn  to  dance  and  to  play. 

1  « Phil.  Trans.'  Vol.  39,  p.  24. 


The  Art  of  Experiment. 


This  also  is  the  only  way  of  learning  the  art  of  experi- 
ment. You  must  not,  therefore,  be  daunted  by  your 
clumsiness  at  first ;  you  must  overcome  it,  and  acquire 
ekill  in  the  art  by  repetition. 

In  this  way  you  will  come  into  direct  contact  with 
natural  truth — you  will  think  and  reason  not  on  what 
has  been  said  to  you  in  books,  but  on  what  has  been 
said  to  you  by  Nature.  Thought  springing  from  this 
source  has  a  vitality  not  derivable  from  mere  book- 
knowledge. 

§  4.  Materials  for  Experiment. 

At  this  stage  of  our  labours  we  are  to  provide  ourselves 
with  the  following  materials  : — 

a.  Some  sticks  of  sealing-wax; 

6.  Two  pieces  of  gutta-percha 
tubing,  about  18  inches  long  and 
|  of  an  inch  outside  diameter  ; 

c.  Two   or  three  glass  tubes, 
about  18  inches  long  and  f  of  an 
inch  wide,  closed  at  one  end,  and 
not  too    thin,   lest    they   should 
break  in  your  hand  and  cut  it ; 

d.  Two   or   three    pieces    of 
clean    flannel,    capable    of  being 
folded  into  pads  of  two  or  three 
layers,  about  eight  or  ten  inches 
square ; 

e.  A  couple  of  pads,  composed 
of  three  or   four  layers  of  silk, 
about  eight  or  ten  inches  square ; 

/.  A   board  about    18  inches  square,  and  a  piece 
of  india-rubber ; 


6  Lessons  in  Electricity. 

g.  Some  very  narrow  silk  ribbon,  E,  and  a  wire  loop, 
W,  like  that  shown  in  fig.  1,  in  which  sticks  of  sealing- 
wax,  tubes  of  gutta-percha,  rods  of  glass,  or  a  walking- 
stick,  may  be  suspended.  I  choose  a  narrow  ribbon 
because  it  is  convenient  to  have  a  suspending  cord  that 
will  neither  twist  nor  untwist  of  itself. 

(I  usually  employ  a  loop  with  the  two  ends,  which 
are  here  shown  free,  soldered  together.  The  loop 
would  thus  be  unbroken.  But  you  may  not  be  skilled 
in  the  art  of  soldering,  and  I  therefore  choose  the  free 
loop,  which  is  very  easily  constructed.  For  the  purpose 
of  suspension  an  arrangement  resembling  a  towel- 
horse,  with  a  single  horizontal  rail,  will  be  found  con- 
venient.) 

Fro.  2. 


h.  A  straw,  I  i',  fig.  2,  delicately  supported  on  the 
point  of  a  sewing  needle  N.  This  is  inserted  in  a 
stick  of  sealing-wax  A,  attached  below  to  a  little 
circular  plate  of  tin,  the  whole  forming  a  stand.  In 
fig.  3  the  straw  is  shown  on  a  larger  scale,  and  separate 


Materials  for  Experiment.  7 

from  its  needle      The  short  bit  of  straw  in  the  middle, 
which  serves  as  a  cap,  is  stuck  on  by  sealing-wax. 


Fro.  3. 

•ar- — 9 


i.  The  name  '  amalgam '  is  given  to  a  mixture  of 
mercury  with  other  metals.  Experience  has  shown 
that  the  efficacy  of  a  silk  rubber  is  vastly  increased 
when  it  is  smeared  over  with  an  amalgam  formed  of 
1  part  by  weight  of  tin,  2  of  zinc,  and  6  of  mercury. 
A  little  lard  is  to  be  first  smeared  on  the  silk,  and  the 
amalgam  is  to  be  applied  to  the  lard.  The  amalgam, 
if  hard,  must  be  pounded  or  bruised  with  a  pestle  or  a 
hammer  until  it  is  soft.  You  can  purchase  sixpenny- 
worth  of  it  at  a  philosophical  instrument  maker's.  It 
is  to  be  added  to  your  materials. 

k.  I  should  like  to  make  these  pages  suitable  for 
boys  without  much  pocket-money,  and,  therefore,  aim 
at  economy  in  my  list  of  materials.  But  provide  by  all 
means,  if  you  can,  a  fox's  brush,  such  as  those  usually 
employed  in  dusting  furniture. 


Lessons  in  Electricity. 


§  5.  Electric  Attractions. 

Place  your  sealing-wax,  gutta-percha  tubing,  and 
flannel  and  silk  rubbers  before  a  fire,  to  ensure  their 
dryness.  Be  specially  careful  to  make  your  glass  tubes 
and  silk  rubbers  not  only  warm,  but  hot.  Pass  the 
dried  flannel  briskly  once  or  twice  over  a  stick  of  seal- 
ing-wax or  over  a  gutta-percha  tube.  A  very  small 
amount  of  friction  will  excite  the  power  of  attract- 
ing the  suspended  straw,  as  shown  in  fig.  2.  Repeat 
the  experiment  several  times  and  cause  the  straw  to 
follow  the  attracting  body  round  and  round.  Do  the 
same  with  a  glass  tube  rubbed  with  silk. 

I  lay  particular  stress  on  the  heating  of  the  glass 
tube,  because  glass  has  the  power,  which  it  exercises, 
of  condensing  upon  its  surface  into  a  liquid  film,  the 
aqueous  vapour  of  the  surrounding  air.  This  film  must 
be  removed. 

I  would  also  insist  on  practice,  in  order  to  render 
you  expert.  You  will  therefore  attract  bran,  scraps  of 
paper,  gold  leaf,  soap  bubbles,  and  other  light  bodies 
by  rubbed  glass,  sealing-wax,  and  gutta-percha.  Fara- 
day was  fond  of  making  empty  egg-shells,  hoops  of 
paper,  and  other  light  objects  roll  after  his  excited 
tubes. 

It  is  only  when  the  electric  power  is  very  weak, 
that  you  require  your  delicately  suspended  straw.  With 
the  sticks  of  wax,  tubes,  and  rubbers  here  mentioned, 
even  heavy  bodies,  when  properly  suspended,  may  be 
attracted.  Place,  for  instance,  a  common  walking-stick 
in  the  wire  loop  attached  to  the  narrow  ribbon,  fig.  1, 
and  let  it  swing  horizontally.  The  glass,  rubbed  with 


Electric  Attractions. 


9 


its  silk,  or  the  sealing-wax,   or  gutta-percha,  rubbed 
with   its  flannel,  will  pull  the  stick  quite  round. 

Abandon  the  wire  loop ;  place  an  egg  in  an  egg-cup, 
and  balance  a  long  lath  upon  the  egg,  as  shown  in  fig.  4. 
The  lath,  though  it  may  be  almost  a  plank,  will  ol)e- 

Fio.  4. 


iliently  follow  the  rubbed  glass,  gutta-percha  or  sealing- 
wax. 

Nothing  can  be  simpler  than  this  lath  and  egg 
arrangement,  and  hardly  anything  could  be  more  im- 
pressive. The  more  you  work  with  it,  the  better  you 
will  like  it. 

Pass  an  ebonite  comb  through  the  hair.  In  dry 
weather  it  produces  a  crackling  noise ;  but  its  action 
upon  the  lath  may  be  made  plain  in  any  weather.  It 
is  rendered  electrical  by  friction  against  the  hair,  and 
with  it  you  can  pull  the  lath  quite  round. 

If  you  moisten  the  hair  with  oil,  the  comb  will  still 
be  excited  and  exert  attraction  ;  but  if  you  moisten  it 
with  water,  the  excitement  ceases;  a  comb  passed 
through  wetted  hair,  has  no  power  over  the  lath.  You 
will  understand  the  meaning  of  this  subsequently. 


10  Lessons  in  Electricity. 

After  its  passage  through  dry  or  oiled  hair,  balance 
the  comb  itself  upon  the  egg :  it  is  attracted  by  the 
lath.  You  thus  prove  the  attraction  to  be  mutual:  the 
comb  attracts  the  lath,  and  the  lath  attracts  the  comb. 
Suspend  your  rubbed  glass,  rubbed  gutta-percha,  and 
rubbed  sealing-wax  in  your  wire  loop.  They  are  all 
just  as  much  attracted  by  the  lath  as  the  lath  was  at- 
tracted by  them.  This  is  an  extension  of  Boyle's  ex- 
periment with  the  suspended  amber  (§  2). 

How  it  is  that  any  unelectri6ed  body  attracts,  and 
is  attracted  by  the  excited  glass,  sealing-wax  and  gutta- 
percha,  we  shall  learn  by  and  by. 

A  very  striking  illustration  of  electric  attraction  may 
be  obtained  with  the  board  and  india-rubber  mentioned 
in  our  list  of  materials  (§  4).  Place  the  board  before 
the  fire  and  make  it  hot ;  heat  also  a  sheet  of  foolscap 
paper  and  place  it  on  the  board.  There  is  no  attraction 
between  them.  Pass  the  india-rubber  briskly  over  the 
paper.  It  now  clings  firmly  to  the  board.  Tear  it 
away,  and  hold  it  at  arm's  length,  for  it  will  move  to 
your  body  if  it  can.  Bring  it  near  a  door  or  wall,  it 
will  cling  tenaciously  to  either.  The  electrified  paper 
also  powerfully  attracts  the  balanced  lath  from  a  great 
distance. 

The  friction  of  the  hand,  of  a  cambric  handkerchief, 
or  of  wash-leather  fails  to  electrify  the  paper  in  any 
high  degree.  It  requires  friction  by  a  special  substance 
to  make  the  excitement  strong.  This  we  learn  by  ex- 
perience. It  is  also  experience  that  has  taught  us  that 
resinous  bodies  are  best  excited  by  flannel,  and  vitreous 
bodies  by  silk. 

Take  nothing  for  granted  in  this  enquiry,  and 
neglect  no  effort  to  render  your  knowledge  complete 


Electric  Attractions.  11 

and  sure.  Try  various  rubbers,  and  satisfy  yourself 
that  differences  like  that  first  observed  by  Newton  exist 
between  them. 

Vary  also  the  body  rubbed.  Excite  by  friction 
paraffin  and  composite  candles,  resin,  sulphur,  bees'- 
wax,  ebonite,  and  shellac.  Also  rock-crystal  and  other 
vitreous  substances,  and  attract  with  all  of  them  the 
balanced  lath.  A  film  of  collodion,  a  sheet  of  vulcan- 
ised india-rubber,  or  brown  paper  heated  before  the 
fire,  rubbed  briskly  with  the  dry  hand,  attracts  and  is 
attracted  by  the  lath. 

Lay  bare  also  the  true  influence  of  heat  in  the  case 
of  our  rubbed  paper.  Spread  a  cold  sheet  of  foolscap 
on  a  cold  board — on  a  table,  for  example.  If  the  air  be 
not  very  dry,  rubbing,  even  with  the  india-rubber,  will 
not  make  them  cling  together.  But  is  it  because  they 
were  hot  that  they  attracted  each  other  in  the  first  in- 
stance ?  No,  for  you  may  heat  your  board  by  plunging  it 
into  boiling  water,  and  your  paper  by  holding  it  in  a 
cloud  of  steam.  Thus  heated  they  cannot  be  made  to 
cling  together.  The  heat  really  acts  by  expelling  the 
moisture.  Cold  weather,  if  it  be  only  dry,  is  highly 
favourable  to  electric  excitation.  During  frost  the 
whisking  of  the  hand  over  silk  or  flannel,  or  over  a  cat's 
back,  renders  it  electrical. 

The  experiment  of  the  Florentine  academicians, 
whereby  they  proved  the  electric  attraction  of  a  liquid, 
is  pretty,  and  worthy  of  repetition.  P"ill  a  very  small 
watch-glass  with  oil,  until  the  liquid  forms  a  round 
curved  surface,  rising  a  little  over  the  rim  of  the  glass. 
A  strongly  excited  glass  tube,  held  over  the  oil,  raises 
not  one  eminence  only,  but  several,  each  of  which 
finally  discharges  a  shower  of  drops  against  the  attract- 


12  Lessons  in  Electricity. 

ing  glass.     The  effect  is  shown  in  fig.  5,  where  G  is  the 

watch-glass  on  the  stand  T,  and  u  the  excited  glass  tube.1 

Cause  the  excited  glass  tube  to  pass  close  by  your 

face,  without  touching  it.   You  feel,  like  Hauksbee,  as  if 

FIG.  5. 


a  cobweb  were  drawn  over  the  face.  You  also  sometimes 
smell  a  peculiar  odour,  due  to  a  substance  developed 
by  the  electricity,  and  called  ozone. 

Long  ere  this,  while  rubbing  your  tubes,  you  will 
have  heard  the  '  hissing'  and  '  crackling'  so  often  re- 
ferred to  by  the  earlier  electricians ;  and  if  you  have 
rubbed  your  glass  tube  briskly  in  the  dark,  you  will  have 
seen  what  they  called  the  '  electric  fire.'  Using,  instead 
of  a  tube,  a  tall  glass  jar,  rendered  hot,  a  good  warm 
rubber,  and  vigorous  friction,  the  streams  of  electric 
fire  are  very  surprising  in  the  dark. 

1  As  a  practical  measure  the  watch-glass  ought  to  rest  upon  a  small 
stand,  and  not  upon  a  surface  of  large  area.  The  eTperiment  is  par- 
ticularly veil  suited  for  projection  on  a  screen. 


Discovery  of  Conduction  and  Insulation.      13 


§  6.  Discovery  of  Conduction  and  Insulation. 

Here  I  must  again  refer  to  that  most  meritorious 
philosopher  Stephen  Gray.  In  1729,  he  experimented 
with  a  glass  tube  stopped  by  a  cork.  When  the  tube 
was  rubbed,  the  cork  attracted  light  bodies.  Gray 
states  that  he  was  '  much  surprised '  at  this,  and  he 
*  concluded  that  there  was  certainly  an  attractive  virtue 
communicated  to  the  cork.'  This  was  the  starting 
point  of  our  knowledge  of  electric  Conduction. 

A  fir  stick  4  inches  long,  stuck  into  the  cork,  was 
also  found  by  Gray  to  attract  light  bodies.     He  made 
his  sticks  longer,  but  still  found      .          Fia.  6. 
a   power    of  attraction    at    their 
ends.  He  then  passed  on  to  pack- 
thread   and    wire.      Hanging    a 
thread  s,  fig.  6,  from  the  top  win- 
dow of  a  house,  so  that  the  lower 
end  nearly  touched   the  ground, 
and  twisting  the  upper  end  of  the 
thread   round   his  glass   tube  R, 
on  briskly  rubbing  the  tube,  light 
bodies  were  attracted  by  the  lower 
end  B  of  the  thread. 

But  Gray's  most  remarkable 
experiment  was  this: — He  sus- 
pended a  long  hempen  line  hori- 
zontally by  loops  of  packthread, 
but  failed  to  transmit  through  it 
the  electric  power.  He  then  sus- 
pended it  by  loops  of  silk  and 
succeeded  in  sending  the  ;  attrac- 
tive virtue'  through  765  feet  of  thread.  He  at  first 


14  Lessons  in  Electricity. 

thought  the  silk  was  effectual  because  it  was  thin  : 
but  on  replacing  a  broken  silk  loop  by  a  still  thinner 
wire,  he  obtained  no  action.  Finally,  he  came  to  the 
conclusion  that  his  loops  were  effectual,  not  because 
they  were  thin  but  because  they  were  silk.  This  was 
the  starting  point  of  our  knowledge  of  Insulation. 

It  is  interesting  to  notice  the  devotion  of  some  men 
of  science  to  their  work.  Dr.  Wells,  who  wrote  a 
beautiful  essay,  wherein  he  explained  the  origin  of 
dew,  finished  it  when  he  was  on  the  brink  of  the 
grave.  Stephen  Gray  was  so  near  dying  when  his  last 
experiments  were  made,  that  he  was  unable  to  write 
out  an  account  of  them.  On  his  death-bed,  and  indeed 
the  very  day  before  his  death,  his  description  of  them 
was  taken  from  his  lips  by  Dr.  Mortimer,  Secretary  of 
the  Royal  Society,  and  afterwards  printed  in  the  '  Philo- 
sophical Transactions.' 

One  word  of  definition  will  be  useful  here.  Some 
substances,  as  proved  by  Stephen  Gray,  possess  in  a  very 
high  degree  the  power  of  permitting  electricity  to  pass 
through  them ;  other  substances  stop  the  passage  of 
the  electricity.  Bodies  of  the  first  class  are  called 
conductors :  bodies  of  the  second  class  are  called 
insulators. 

You  cannot  do  better  than  repeat  here  the  experi- 
ments of  Gray.  Push  a  cork  into  the  open  end  of 
your  glass  tube  ;  rub  the  tube,  carrying  the  friction 
up  to  the  end  holding  the  cork.  The  cork  will  attract 
the  balanced  lath,  shown  in  rig.  4,  with  which  you  have 
already  worked  so  much. 

But  the  excited  glass  is  here  so  near  the  end  of 
the  cork  that  you  may  not  feel  certain  that  the  observed 
attraction  is  that  of  the  cork.  You  can,  however,  prove 


Discovery  of  Conduction  and  Insulation. 


13 


that  the  cork  attracts  by  its  action  upon  light  bodies 
which  cling  to  it.  Stick  a  pen-holder  into  the  cork,  and 
rub  the  glass  tube  as  before.  The  free  end  of  the  holder 
will  attract  the  lath.  Stick  a  deal  rod  three  or  four 
feet  long  into  the  cork  ;  its  free  end  will  attract  the 
lath  when  the  glass  tube  is  excited.  In  this  way,  you 
prove  to  demonstration  that  the  electric  power  is  con- 
veyed along  the  rod. 


§  7.    The  Electroscope.     Further  Enquiries  on  Con- 
duction and  Insulation. 

A  little  addition  to  our  apparatus  will  now  be 
desirable.  You  can  buy  a  book  of  '  Dutch  metal '  for 
fourpence  ;  and  a  globular  flask  like  that  shown  in  fig.  7, 

FIG.  7. 


for  sixpence,  or  at  the  most  a  shilling.  Find  a  cork, 
C,  which  fits  the  flask ;  pass  a  wire,  w,  through  the  cork 
and  bend  it  near  one  end  at  a  right  angle.  Attach 


1 6  Lessons  in  Electricity. 

by  means  of  wax  to  the  bent  arm,  which  ought  to  be 
about  three  quarters  of  an  inch  long,  two  strips,  L,  of 
the  Dutch  metal,  about  three  inches  long  and  from 
half  an  inch  to  three-quarters  of  an  inch  wide.  The 
strips  will  hang  down  face  to  face,  in  contact  with 
each  other.  Stick  by  sealing-wax  upon  the  other  end 
of  the  wire  a  little  plate  of  tin  or  sheet-zinc,  T,  about 
two  inches  in  diameter.  In  all  cases  you  muct  be 
careful  so  to  use  your  wax  as  not  to  interrupt  the  metallic 
connection  of  the  various  parts  of  your  apparatus,  which 
we  will  name  an  electroscope.  Gold  leaf,  instead  of 
Dutch  metal,  is  usually  employed  for  electroscopes.  I 
recommend  the  'metal'  because  it  is  cheaper,  and 
will  stand  rougher  usage. 

See  that  your  globular  flask  is  dry  and  free  from 
dust.  Bring  your  rubbed  sealing-wax,  E,  or  your  rubbed 
glass,  near  the  little  plate  of  tin,  the  leaves  of  Dutch 
metal  open  out ;  withdraw  the  excited  body,  the  leaves 
fall  together.  We  shall  enquire  into  the  cause  of  this 
action  immediately.  Practise,  the  approach  and  with- 
drawal for  a  little  time.  Now  draw  your  rubbed  sealing- 
wax  or  glass  along  the  edge  of  the  tin  plate,  T.  The  leaves 
diverge,  and  after  the  sealing-wax  or  glass  is  withdrawn 
they  remain  divergent.  In  the  first  experiment  you  com- 
municated no  electricity  to  the  electroscope ;  in  the 
second  experiment  you  did.  At  present  I  will  only  ask 
you  to  take  the  opening  out  of  the  leaves  as  a  proof 
that  electricity  has  been  communicated  to  them. 

And  now  we  are  ready  for  Gray's  experiments  in 
a  form  different  from  his.  Connect  the  end  of  a  long 
wire  with  the  tin  plate  of  the  electroscope  ;  coil  the  other 
end  round  your  glass  tube.  Eub  the  tube  briskly,  car- 
rying the  friction  close  to  the  coiled  wire.  A  single 


Further  Enquiries  on  Conduction,  &c.         17 

stroke  of  your  rubber,  if  skilfully  given,  will  cause  the 
leaves  to  diverge.  The  electricity  has  obviously  passed 
through  the  wire  to  the  electroscope. 

Substitute  for  the  wire  a  string  of  common  twine, 
rub  briskly  and  you  will  cause  the  leaves  to  diverge ; 
but  there  is  a  notable  difference  as  regards  the  prompt- 
ness of  the  divergence.  You  soon  satisfy  yourself  that 
the  electricity  passes  with  greater  facility  through  the 
wire  than  through  the  string.  Substitute  for  the 
twine  a  string  of  silk.  Mo  matter  how  vigorously  you 
rub  you  can  now  produce  no  divergence.  The  elec- 
tricity cannot  get  through  the  silk  at  all. 

This  is  the  place  to  demonstrate  in  a  manner  never 
to  be  forgotten  the  influence  of  moisture.  Wet 
your  dry  silk  string  throughout,  and  squeeze  it  a  little 
so  that  the  water  from  it  may  not  trickle  over  your 
glass  tube.  Coil  it  round  the  tube  as  before,  and 
excite  the  tube.  The  leaves  of  the  electroscope  imme- 
diately diverge.  The  water  is  here  the  conductor.  The 
influence  of  moisture  was  first  demonstrated  by  Du  Fay 
(1733  to  1737),  who  succeeded  in  sending  electricity 
through  1,256  feet  of  moist  packthread. 

It  is  hardly  necessary  to  point  out  the  meaning  of 
Gray's  experiment  where  he  found  that,  with  loops  of 
wire  or  of  packthread,  he  could  not  send  the  electricity 
from  end  to  end  of  his  suspended  string.  Obviously 
the  electricity  escaped  in  each  of  these  cases  through 
the  conducting  support  to  the  earth. 

My  assistant,  Mr.  Cottrell,  who  has  been  working  very 
hard  for  you  and  me,  has  devised  an  electroscope  which 
we  shall  frequently  employ  in  our  lessons.  M,  fig.  8,  is 
a  little  plate  of  metal,  or  of  wood  covered  with  tin-foil, 
supported  on  a  rod  G  of  glass  or  of  sealing-wax.  N  is 


18  Lessons  in  Electricity. 

another  plate  of  Dutch  metal  paper,  separated  about  an 
inch  from  M.  and  attached  by  sealing-wax  to  the  long 

FIG,  8. 
I  ait         y~\ 


straw  1 1'  (broken  off  in  the  figure) ;  A  A'  is  a  horizontal 
pivot  formed  by  a  sewing  needle,  and  supported  on  a  bent 
strip  of  metal,  as  shown  in  the  figure.  By  weighting  the 
straw  with  a  little  wire  near  i',  you  so  balance  it  that  the 
plate  N  shall  be  just  lifted  away  from  M.  The  wire  w, 
which  may  be  100  feet  long,  proceeds  from  M  to  your 
glass  tube,  round  which  it  is  coiled.  A  single  vigorous 
stroke  of  the  tube  by  the  rubber  sends  electricity  along 
w  to  M  ;  N  is  attracted  downwards,  the  other  end  of  the 
long  straw  being  lifted  through  a  considerable  distance. 
In  subsequent  figures  you  will  see  the  complete  straw- 
index,  and  its  modes  of  application. 

A  few  experiments  with  either  of  these  instruments 
will  enable  you  to  classify  bodies  as  conductors,  semi- 
conductors, and  insulators.  Here  is  a  list  of  a  few  of 
each,  which,  however,  differ  much  among  themselves. 

Conductors. 

The  common  metals 
Well  burned  charcoal 
Concentrated  acids 
Solutions  of  salts 


Further  Enquiries  on  Conduction,  &c.         19 

Eain  water 

Linen 

Living  vegetables  and  animals. 

Semi- conductors. 

Alcohol  and  efcher  Paper 

Dry  wood  Straw. 

Marble 

Insulators. 

Fatty  oils  Silk 

Chalk  Glass 

India-rubber  Wax 

Dry  paper  Sulphur 

Hair  Shellac. 

A  little  reflection  will  enable  you  to  vary  these 
experiments  indefinitely.  Kub  your  excited  sealing- 
wax  or  glass  against  the  tin  plate  of  your  electroscope, 
and  cause  the  leaves  to  diverge.  Touch  the  plate  with 
any  one  of  the  conductors  mentioned  in  the  list ;  the 
electroscope  is  immediately  discharged.  Touch  it  witli 
a  semi -conductor ;  the  leaves  fall  as  before,  but  less 
promptly.  Touch  the  plate  finally  with  an  insulator,  the 
electricity  cannot  pass,  and  the  leaves  remain  unchanged. 

§  8.  Electrics  and  Non-Electrics. 

For  a  long  period,  bodies  were  divided  into  electrics 
and  non-electrics,  the  former  deemed  capable  of  being 
electrified,  the  latter  not.  Thus  the  amber  of  the  an- 
cients, and  the  spars,  gems,  fossils,  stones,  glasses,  and 
resins,  operated  on  by  Dr.  Gilbert,  were  called  electrics, 


20  Lessons  ii  Electricity. 

while  all  tire  metals  were  called  non-electrics.  We  must 
now  determine  the  true  meaning  of  this  distinction. 

Take  in  succession  a  piece  of  brass,  of  wood  coated 
with  tin-foil,  a  lead  bullet,  apples,  pears,  turnips,  car- 
rots, cucumbers — un coated  wood  not  very  dry  will  also 
answer — in  the  hand,  arid  strike  them  briskly  with 
flannel,  or  the  fox's  brush  ;  none  of  them  will  attract 
the  balanced  lath,  fig.  4,  or  show  any  other  symptom 
of  electric  excitement.  All  of  them  therefore  would 
have  been  once  called  non-electrics. 

But  suspend  them  in  succession  by  a  string  of  silk 
held  in  the  hand,  and  strike  them  again ;  every  one 
of  them  will  now  attract  the  lath. 

Eeflect  upon  the  meaning  of  this  experiment.  We 
have  introduced  an  insulator — the  silk  string — be- 
tween the  hand  and  the  body  struck,  and  we  find  that 
by  its  introduction  the  non-electric  has  been  converted 
into  an  electric. 

The  meaning  is  obvious.  When  held  in  the  hand, 
though  electricity  was  developed  in  each  case  by  the 
friction,  it  passed  Immediately  through  the  hand  and 
body  to  the  earth.  This  transfer  being  prevented  by 
the  silk,  the  electricity,  once  excited,  is  retained,  and 
the  attraction  of  the  lath  is  the  consequence. 

In  like  manner,  a  brass  tube,  held  in  the  hand  and 
struck  with  a  fox's  brush,  shows  no  attractive  power  ; 
but  when  a  stick  of  sealing-wax,  ebonite,  or  gutta- 
percha  is  thrust  into  the  tube  as  a  handle,  the  striking 
of  the  tube  at  once  develops  the  power  of  attraction. 

And  now  you  see  more  clearly  than  you  did  at  first 
the  meaning  of  the  experiment  with  the  heated 
foolscap  and  india-rubber.  Paper  and  wood  always 
imbibe  a  certain  amount  of  moisture  from  the  air. 


Electrics  and  Non-Electrics.  21 

When  the  rubber  was  passed  over  the  cold  paper  elec- 
tricity was  excited,  but  the  paper,  being  rendered  a 
conductor  by  its  moisture,  allowed  the  electricity  to 
pass  away. 

Prove  all  things.  Lay  your  cold  foolscap  on  a  cold 
board  supported  by  dry  tumblers :  pass  your  india- 
rubber  over  the  paper ;  lift  it  by  a  loop  of  silk,  which 
has  been  previously  attached  to  it,  for  if  you  touch 
it  it  will  discharge  itself.  You  will  find  it  electric  ; 
and  with  it  you  can  charge  your  electroscope,  or  attr ict 
from  a  distance  your  balanced  lath. 

The  human  body  was  ranked  among  the  non-elec- 
trics. Make  plain  to  yourself  the  reason.  Stand  upon 
the  floor  and  permit  a  friend  to  strike  you  briskly  with 
the  fox's  brush.  Present  your  knuckle  to  the  balanced 
lath,  you  will  find  no  attraction.  Here,  however,  you 
stand  upon  the  earth,  so  that  even  if  electricity  had 
been  developed,  there  is  nothing  to  hinder  it  from 
passing  away. 

But,  place  upon  the  ground  four  warm  glass 
tumblers,  and  upon  the  tumblers  a  board.1  Stand  upon 
the  board,  and  present  your  knuckle  to  the  lath.  A 
single  stroke  of  the  fox's  fur,  if  skilfully  given,  will 
produce  attraction.  If  you  stand  upon  a  cake  of  resin, 
of  ebonite,  or  upon  a  sheet  of  good  india-rubber,  the 
effect  will  be  the  same.  You  can  also  charge  your 
electroscope  with  this  electricity. 

Throw  a  mackintosh  over  your  shoulders  and  let  a 
friend  strike  it  with  the  fox's  brush,  the  attractive  force 
is  greatly  augmented. 

1  Some  caution  is  necessary  here.  A  large  class  of  cheap  glass 
tumblers  conduct  so  freely  that  they  are  unfit  for  this  and  similar  ex- 
periments See  §  19. 


22  Lessons  in  Electricity. 

After  brisk  striking,  present  your  knuckle  to  the 
knuckle  of  your  friend.  A  spark  will  pass  between  you. 

This  experiment  with  the  mackintosh  further  illus- 
trates what  you  nave  already  frequently  observed,namely, 
that  it  is  not  friction  alone,  but  the  friction  of  special 
substances  against  each  other,  that  produces  electricity. 

Thus  we  prove  that  non-electrics,  like  electrics,  can 
be  excited,  the  condition  of  success  being,  that  an  insu- 
lator shall  be  interposed  between  the  non-electric  and 
the  earth.  It  is  obvious  that  the  old  division  into 
electrics  and  non-electrics,  really  meant  a  division  into 
insulators  and  conductors. 


§  9.  Electric  Repulsions.  Discovery  of  two  Electricities 

We  have  hitherto  dealt  almost  exclusively  with 
electric  attractions,  but  in  an  experiment  already  re- 
ferred to  (§  2),  Otto  von  Gruericke  observed  the  repul- 
sion of  a  feather  by  his  sulphur  globe.  I  also  antici- 
pated matters  in  the  use  of  our  Dutch  metal  electroscope 
(§  7),  where  the  repulsion  of  the  leaves  informed  us 
of  the  arrival  of  the  electricity. 

Du  Fay,  who  was  the  real  discoverer  here,  found  a 
gold-leaf  floating  in  the  air  to  be  fir.-t  attracted  and 
then  repelled  by  the  same  excited  body.  He  afterwards 
proved  that  when  thefloating  leaf  was  repelled  by  rubbed 
glass,  it  was  attracted  by  rubbed  resin, — and  that  when 
it  was  repelled  by  rubbed  resin,  it  was  attracted  by 
rubbed  glass.  Hence  the  important  announcement,  by 
Du  Fay,  that  there  are  two  kinds  of  electricity. 

The  electricity  excited  on  glass  was  for  a  time 
called  vitreous  electricity,  —  while  that  excited  on 
sealing-wax  was  called  resinous  electricity.  These 


Discovery  of  two  Electricities.  23 

terms  are  however  improper ;  because,  by  changing  the 
rubber,  we  can  obtain  the  electricity  of  sealing-wax 
upon  glass,  and  the  electricity  of  glass  upon  sealing- 
wax. 

Eoughen,  for  example,  the  surface  of  your  glass  tube 
with  a  grindstone,  and  rub  it  with  flannel,  the  electricity 
of  sealing-wax  will  be  found  upon  the  vitreous  surface. 
Rub  your  sealing-wax  with  vulcanised  india-rubber,  the 
electricity  of  glass  will  be  found  upon  the  resinous 
surface.  You  will  be  able  to  prove  this  immediately. 

"We  now  use  the  term  positive  or  plus  electricity  to 
denote  that  developed  on  glass  bythe  friction  of  silk;  and 
negative  or  minus  electricity  to  denote  that  developed 
on  sealing-wax  by  the  friction  of  flannel.  These  terms 
are  adopted  purely  for  the  sake  of  convenience.  There  is 
no  reason  in  nature  why  the  resinous  electricity  should 
not  be  called  positive,  and  the  vitreous  electricity  nega- 
tive. Once  agreed,  however,  to  apply  the  terms  as  here 
fixed,  we  must  adhere  to  this  agreement  throughout. 

§  10.  Fundamental  Law  of  Electric  Action. 

In  all  the  experiments  which  we  have  hitherto  made, 
one  of  the  substances  operated  on  has  been  electrified 
by  friction,  and  the  other  not.  But  once  engaged  in 
enquiries  of  this  description,  questions  incessantly  occur 
to  the  mind,  the  answering  of  which  extends  our  know- 
ledge, and  suggests  other  questions.  Suppose,  instead  of 
exciting  only  one  of  the  bodies  presented  to  each  other, 
we  were  to  excite  both  of  them,  what  would  occur  ?  This 
is  the  question  which  was  asked  and  answered  by  Du 
Fay,  and  which  we  must  now  answer  for  ourselves. 

Here  your  wire  loop,  fig.  1,  comes  again  into  play 


24  Lessons  in  Electricity. 

Place  an  un-rubbed  gutta-percha  tube,  or  a  stick  of 
sealing-wax,  in  the  loop,  and  be  sure  that  it  is  un-rubbed 
— that  no  electricity  adheres  to  it  from  former  experi- 
ments. If  it  fail  to  attract  light  bodies,  it  is  unexcited  ; 
if  it  attract  them,  pass  your  hand  over  it  several  times, 
or,  better  still,  pass  it  over  or  through  the  flame 
of  a  spirit  lamp.  This  will  remove  every  trace  of 
electricity.  Satisfy  yourself  that  the  un-rubbed  gutta- 
percha  tube  is  attracted  by  a  rubbed  one. 

Eemove  the  un-rubbed  tube  from  the  loop,  and  ex- 
cite it  with  its  flannel  rubber.  One  end  of  the  tube  is 
held  in  your  hand  and  is  therefore  unexcited.  Eeturn 
the  tube  to  the  loop,  keeping  your  eye  upon  the  excited 
end.  Bring  a  second  rubbed  tube  near  the  excited  end 
of  the  suspended  one :  strong  repulsion  is  the  conse- 
quence. Drive  the  suspended  tube  round  and  round 
by  this  force  of  repulsion. 

Bring  a  rubbed  glass  tube  near  the  excited  end  of 
the  gutta-percha  tube  :  strong  attraction  is  the  result. 

Repeat  this  experiment  step  by  step  with  two 
glass  tubes.  Prove  that  the  rubbed  glass  tube  attracts 
the  un-rubbed  one.  Remove  the  un-rubbed  tube  from 
the  loop,  excite  it  by  its  rubber,  return  it  to  the  loop, 
and  establish  the  repulsion  of  glass  by  glass.  Bring 
rubbed  gutta-percha  or  sealing-wax  near  the  rubbed 
glass :  strong  attraction  is  the  consequence. 

These  experiments  lead  you  directly  to  the  funda- 
mental law  of  electric  action,  which  is  this : — Bodies 
charged  with  the  same  electricity  repel  each  other, 
while  bodies  charged  with  opposite  electricities  attract 
each  other.  Positive  repels  positive,  and  attracts  nega~ 
tive.  Negative  repels  negative,  and  attracts  positive. 

Devise  experiments  which  shall  still  further  illus- 


Fundamental  Law  of  Electric  Action.         25 

trate  this  law.  Repeat,  for  example,  Otto  von  Gruericke's 
experiment.  Hang  a  feather  by  a  silk  thread  and, 
bring  your  rubbed  glass  tube  near  it :  the  feather  is 
attracted,  touches  the  tube,  charges  itself  with  the  elec- 
tricity of  the  tube,  and  is  then  repelled.  Cause  it  to 
retreat  from  the  tube  in  various  directions.  Du  Fay's 
experiment  with  he  gold-leaf  will  be  repeated  and 
explained  further  on.  See  §  18. 

Hang  your  feather  by  a  common  thread  :  if  no  insu- 
lating substance  intervenes  between  the  feather  and  the, 
earth,  you  can  get  no  repulsion.  Why  ?  Obviously 
because  the  charge  of  positive  electricity  communicated 
by  the  rod,  is  not  retained  by  the  feather,  but  passes 
away  to  the  earth.  Hence,  you  have  not  positive  acting 
against  positive  at  all.  Why  the  neutral  body  is  at- 
tracted by  the  electrified  one,  will,  as  already  stated, 
appear  by  and  \>y. 

Attract  your  straw  needle  by  your  rubbed  glass 
tube.  Let  the  straw  strike  the  tube,  so  that  the  one 

FIG.  9. 


1 1 

«ball  rub  against  the  other.     The  straw  accepts 


2(5  Lessons  in  Electricity. 

electricity  of  the  tube  and  repulsion  immediately  follows 
attraction,  as  shown  in  fig.  9. 

Mr.  Cottrell  has  devised  the  simple  electroscope 
represented  in  fig.  10  to  show  repulsion.  A  is  a  stem  of 
sealing-wax  with  a  small  circle  of  tin  T  at  the  top.  w 
is  a  bent  wire  proceeding  from  T,  with  a  small  disk 
attached  to  it  by  wax.  i  i'  is  a  little  straw  index, 
supported  by  the  needle  N,  as  shown  in  fig.  10.  The 
stem  A',  also  of  sealing-wax,  is  not  quite  vertical,  the 
ooject  being  to  cause  the  bit  of  paper  V,  to  rest  close 

FIG.  10. 


to  w  when  the  apparatus  is  not  electrified.  When 
electricity  is  imparted  to  T,  it  flows  through  the  wires 
w  and  w,  over  both  disk  and  index  :  immediate  repul- 
sion of  the  straw  is  the  consequence. 

No  better  experiment  can  be  made  to  illustrate  the 
self-repulsive  character  of  electricity  than  the  follow- 
ing one.  Heat  your  square  board  (§  5),  and  warm, 
as  nefore,  your  sheet  of  foolscap.  Spread  the  paper 


Fundamental  Law  of  Electric  Action.         27 

apon  the  board,  and  excite  it  by  the  friction  of  india- 
rubber.  Cut  from  the  sheet  two  long  strips  with  your 
penknife.  Hold  the  strips  together  at  one  end.  Separate 
them  from  the  board,  and  lift  them  into  the  air :  they 
forcibly  drive  each  other  apart,  producing  a  wide  diver- 
gence. 

Cut  several  strips,  so  as  to  form  a  kind  of  tassel. 
Hold  them  together  at  one  end.     Separate  them  from 

Fia.  11. 


the  board,  and  lift  them  into  the  air  :  they  are  driven 
asunder  by  the  self-repellent  electricity,  presenting  an 
appearance  which  may  remind  you  of  the  hair  of 
Medusa.  The  effect  is  represented  in  fig.  II.1 

1  In  one  of  my  earliest  lectures  at  the  Royal  Institution,  having 
rubbed  a  sheet  of  foolscap,  I  was  about  to  lift  it  bodily  from  the  hot 
board,  and  to  place  it  against  the  wall,  when  the  thought  of  cutting  it 
into  strips,  and  allowing  them  to  act  upon  each  other,  occurred  to  me. 
The  result,  of  course,  was  that  above  described.  Simple  and  obvious  aa 
it  was,  it  pave  Faraday,  who  was  present  at  the  time,  the  most  lively 
pleasure.  The  simplest  experiment,  if  only  suited  to  its  object,  de- 
lighted him. 


28 


Lessons  in  Electricity. 


Another  very  beautiful  experiment  fits  in  here, 
Let  fine  silver  sand,  s,  fig.  12,  issue  in  a  stream  from  a 
glass  funnel,  through  an  aperture  one-eighth  of  an  inch 
in  diameter.  Connect  the  sand  in  the  funnel  by  a  fine 
wire  w,  fig.  13,  with  your  warm  glass  tube.  Uneiectrified, 


Fio.  12. 


FIG   13. 


the  sand  particles  descend  as  a  continuous  stream,  s  s', 
fig.  1 2,  but  at  every  stroke  of  the  rubber  they  fly  asunder, 
as  in  fig.  13,  through  self- repulsion.1 

Or  let  three  or  four  fine  fillets  of  water  issue  from 
three  or  four  pin-holes  in  the  bottom  of  a  vessel  close 

1  For  these,  and  also  for  experiments  "with  the  electroscope,  the 
teacher  of  a  large  class  will  find  the  lime-light  shadows  upon  a  white 
screen  (or  better  still,  those  of  the  electric  light)  exceedingly  useful. 
The  effects  are  thus  rendered  risible  to  all  at  once. 


Fundamental  Law  of  Electric  Action.         29 

to  each  other.  Connect  the  water  of  the  vessel  with 
your  glass  tube,  and  rub  as  before.  The  liquid  veins 
are  scattered  into  spray  by  every  stroke  of  the  rubber. 

These  experiments  are  best  made  with  'Cottrell's 
rubber,'  described  in  §  24. 

And  now  you  must  learn  to  determine  with  cer- 
tainty the  quality  of  the  electricity  with  which  any  body 
presented  to  you  may  be  charged.  You  see  immediately 
that  attraction  is  no  sure  test,  because  un-electrified 
bodies  are  attracted.  Further  on  (§  14)  you  will  be 
able  to  grapple  with  another  possible  source  of  error  in 
the  employment  of  attraction. 

In  determining  quality,  you  must  ascertain,  by 
trial,  the  kind  of  electricity  by  which  the  charged  body 
is  repelled ;  if,  for  example,  any  electrified  body  repel, 
or  is  repelled  by,  sealing-wax  rubbed  with  flannel,  the 
electricity  of  the  body  is  negative ;  if  it  repel,  or  is 
repelled  by,  glass,  rubbed  with  silk,  its  electricity  is 
positive.  Du  Fay  had  the  sagacity  to  propose  this 
mode  of  testing  quality. 

Apply  this  test  to  the  strips  of  foolscap  paper  ex- 
cited by  the  india-  rubber.  Bring  a  rubbed  gutta-percha 
tube  near  the  electrified  strips,  you  have  strong  attrac- 
tion. Bring  a  rubbed  glass  tube  between  the  strips, 
you  have  strong  repulsion  and  augmented  divergence. 
Hence,  the  electricity,  being  repelled  by  the  positive 
glass,  is  itself  positive. 

§  11.  Electricity  of  the  Rubber.     Double  or  '  Polar' 
Character  of  the  Electric  Force. 

"We  have  examined  the  action  of  each  kind  of  elec- 
tricity upon  itself,  and  upon  the  other  kind;  but  hitherto 


50  Lessons  in  Electricity. 

we  have  kept  the  rubber  out  of  view.  One  of  the  ques- 
tions which  inevitably  occur  to  the  enquiring  scientific 
mind  would  oe  :  how  is  the  rubber  affected  by  the  act  of 
friction  ?  Here,  as  elsewhere,  you  must  examine  the 
subject  for  yourself,  and  base  your  conclusions  on  the 
facts  you  establish. 

Test  your  rubber  then,  by  your  balanced  lath.  The 
lath  is  attracted  by  the  flannel  which  has  rubbed  against 
gutta-percha ;  and  it  is  attracted  by  the  silk,  which  has 
rubbed  against  glass. 

Kegarding  the  quality  of  the  electricity  of  the 
flannel  or  of  the  silk  rubber,  the  attraction  of  the  lath 
teaches  you  nothing.  But,  suspend  your  rubbed  glass 
tube,  and  bring  the  flannel  rubber  near  it :  repulsion 
follows.  The  silk  rubber,  on  the  contrary,  attracts  the 
glass  tube.  Suspend  your  rubbed  gutta-percha  tube, 
and  bring  the  silk  rubber  near  it  :  repulsion  follows. 
The  flannel,  on  the  contrary,  attracts  the  tube. 

The  conclusion  is  obvious :  the  electricity  of  the 
flannel  is  positive,  that  of  the  silk  is  negative. 

But  the  flannel  is  the  rubber  of  the  gutta-percha, 
whose  electricity  is  negative ;  and  the  silk  is  the  rubber 
of  the  glass,  whose  electricity  is  positive.  Consequently, 
we  have  not  only  proved  the  rubber  to  be  electrified  by 
the  friction,  but  also  proved  the  electricity  of  the  rubber 
to  be  opposite  in  quality  to  that  of  the  body  rubbed. 

All  your  subsequent  experience  will  verify  the 
statement  that  the  two  electricities  always  go  together; 
that  you  cannot  excite  one  of  them  without  at  the 
same  time  exciting  the  other,  and  that  the  electricity 
of  the  rubber,  though  opposite  in  quality,  is  in  all 
cases  precisely  equal  in  quantity  to  that  of  the  body 
rubbed. 


Electricity  of  the  Rubber.  31 

And  now  we  will  test  these  principles  by  a  nev? 
experiment.  In  §  5  we  learned  that  an  ebonite  comb  is 
electrified  by  its  passage  through  dry  hair.  You  can 
readily  prove  the  electricity  of  the  comb  to  be  negative. 
But  the  hair  is  here  the  rubber,  and,  in  accordance  with 
the  principle  just  laid  down,  an  equal  quantity  of 
positive  electricity  has  been  excited  in  the  hair.  If 
you  stand  on  the  ground  uninsulated,  the  electricity  of 
the  hair  passes  freely  through  your  body  to  the  earth. 

But  stand  upon  an  insulating  stool ' — on  your  board, 
for  example,  supported  by  four  warm  tumblers — while 
I,  standing  on  the  ground,  pass  the  comb  briskly  through 
your  hair.  I  pass  it  ten,  twenty,  thirty  times,  and  then 
ask  you  to  attract  your  balanced  lath.  You  present 
your  knuckle,  but  there  is  no  attraction. 

Here  the  comb  and  the  hair  soon  reach  their  maxi- 
mum excitement,  beyond  which  no  further  development 
of  electricity  occurs.  Now,  though  the  comb,  as  shown 
in  §  5,  is  competent  to  attract  the  lath,  while  your  body 
is  here  incompetent  to  do  so,  this  may  be  because  the 
small  quantity  of  electricity  existing  in  a  concen- 
trated form  upon  the  comb  becomes,  when  diffused  over 
the  body,  too  feeble  to  produce  attraction. 

Can  we  not  exalt  the  electricity  of  your  body? 
Guided  by  the  principles  laid  down,  let  us  try  to  do  so. 
First  I  pass  the  unelectrified  comb  through  your  hair ;  it 
comes  away  electrified.  Alter  discharging  the  comb  by 
passing  my  hand  closely  over  it,  I  pass  it  again  through 
the  hair.  As  before,  it  quits  the  hair  electrified,  and  I 

1  A  stool  with  glass  legs  which,  to  protect  them  from  the  moisture 
of  the  air,  are  usually  coated  with  a  solution  of  shellac.  Kegardir:? 
the  attraction  of  glass  for  atmospheric  humidify,  you  will  call  to  mmJ 
what  has  been  said  in  §  5. 


32  Lessons  in  Electricity. 

again  discharge  it.  I  do  this  ten  or  twenty  times, 
always  depriving  the  comb  of  its  electricity  after  it  has 
quitted  the  hair.  Now  present  your  knuckle  to  the 
balanced  lath.  It  is  powerfully  attracted. 

Here,  as  I  have  said,  the  unelectrified  comb  carried 
in  each  case  electricity  away  with  it ;  but,  in  accordance 
with  the  foregoing  principles,  it  left  an  equal  quantity 
of  the  opposite  electricity  behind  it.  And  though  the 
amount  of  electricity  corresponding  to  a  single  charge 
of  the  comb,  when  diffused  over  the  body,  proved  insen- 
sible to  our  tests,  that  amount  ten  or  twenty  times 
multiplied  became  not  only  sensible,  but  strong.  Indeed, 
by  discharging  the  comb,  and  passing  it  in  each  case 
unelectrified  through  the  hair,  the  insulated  human  body 
can  be  rendered  highly  electrical. 

Near  the  beginning  of  this  section  I  said,  in  rather 
an  off-hand  way,  that  rubbed  flannel  repels  rubbed  glass, 
while  rubbed  silk  repels  rubbed  gutta-percha.  Now, 
while  it  is  generally  easy  to  obtain  the  repulsion  by  the 
flannel,  it  is  by  no  means  always  easy  to  obtain  the 
repulsion  by  the  silk.  Over  and  over  again  I  have 
been  foiled  in  my  attempts  to  show  this  repulsion.  I 
wish  you,  therefore,  to  be  aware  of  an  infallible  method 
of  obtaining  it. 

Stand  on  your  insulating  stool,  and  rub  your  glass 
tube  briskly  with  the  amalgamated  silk  ;  hand  me  the 
tube.  I  pass  my  hand  closely  over  its  surface,  removing 
from  that  surface  nearly  the  whole  of  its  electricity.  I 
hand  you  the  tube  again,  and  you  again  excite  it.  You 
hand  it  to  me,  and  I  again  discharge  it.  In  each  case, 
therefore,  you  excite  an  unelectrified  glass  tube,  and  in 
each  case  the  tube  leaves  behind  upon  the  rubber  an 
amount  of  negative  electricity  equal  in  quantity  to  the 


Double  or  « Polar,1  &c.  33 

positive  carried  away.  By  thus  adding  charge  to 
charge,  the  rubber  is  rendered  highly  electrical ;  aiid, 
even  should  its  insulating  power  be  impaired  by  the 
amalgam,  it  can  now  afford  to  yield  a  portion  of  its 
electricity  to  your  hand  and  body,  and  still  powerfully 
repel  rubbed  gutta-percha.  The  principle,  which 
might  be  further  illustrated,  is  obviously  the  same  as 
that  applied  in  the  case  of  the  comb. 

§  12.   What  is  Electricity* 

Thus  far  we  have  proceeded  from  fact  to  fact,  ac- 
quiring knowledge  of  a  very  valuable  kind.  But  facts 
alone  cannot  satisfy  us.  We  seek  a  knowledge  of  the 
principles  which  lie  behind  the  facts,  and  which  are  to 
be  discerned  by  the  mind  alone.  Thus,  having  spoken 
as  we  have  done,  of  electricity  passing  hither  and 
thither,  and  of  its  being  prevented  from  passing ; 
hardly  any  thoughtful  boy  or  girl  can  avoid  asking 
what  is  it  that  thus  passes  ? — what  is  electricity  ? 
Boyle  and  Newton  betrayed  their  need  of  an  answer  to 
this  question  when  the  one  imagined  his  unctuous 
threads  issuing  from  and  returning  to  the  electrified 
body ;  and  when  the  other  imagined  that  an  elastic 
fluid  existed  which  penetrated  his  rubbed  glass. 

When  I  say  c  imagined  '  I  do  not  intend  to  repre- 
sent the  notions  of  these  great  men  as  vain  fancies. 
Without  imagination  we  can  do  nothing  here.  By 
imagination  I  mean  the  power  of  picturing  mentally 
things  which,  though  they  have  an  existence  as  real  as 
that  of  the  world  around  us,  cannot  be  touched  directly 
by  the  organs  of  sense.  I  mean  the  purified  scientific 
imagination,  without  the  exercise  of  which  we  cannot 


34  Lessons  in  Electricity. 

take  a  single  step  into  the  region  of  causes  and  prin- 
ciples. 

It  was  by  the  exercise  of  the  scientific  imagination 
that  Franklin  devised  the  theory  of  a  single  electric 
fluid  to  explain  electrical  phenomena.  This  fluid  he 
supposed  to  be  self-repulsive,  and  diffused  in  definite 
quantities  through  all  bodies.  He  supposed  that  when 
a  body  has  more  than  its  proper  share  it  is  positively, 
when  less  than  its  proper  share  it  is  negatively  elec- 
trified. It  was  by  the  exercise  of  the  same  faculty  that 
Symmer  devised  the  theory  of  two  electric  fluids,  each 
self-repulsive,  but  both  mutually  attractive. 

At  first  sight  Franklin's  theory  seems  by  far  the 
simpler  of  the  two.  But  its  simplicity  is  only  apparent. 
For  though  Franklin  assumed  only  one  fluid,  he  was 
obliged  to  assume  three  distinct  actions.  Firstly,  he 
had  the  self-repulsion  of  the  electric  particles.  Secondly, 
the  mutual  attraction  of  the  electric  particles  and  the 
ponderable  particles  of  the  body  through  which  the 
electricity  was  diffused.  Thirdly,  these  two  assump- 
tions when  strictly  followed  out  lead  to  the  unavoidable 
conclusion  that  the  material  particles  also  mutually 
repel  each  other.  Thus  the  theory  is  by  no  means  so 
simple  as  it  appears. 

The  theory  of  Symmer,  though  at  first  sight  the 
most  complicated,  is  in  reality  by  far  the  simpler  of  the 
two.  According  to  it  electrical  actions  are  produced 
by  two  fluids,  each  self-repulsive,  but  both  mutually 
attractive.  These  fluids  cling  to  the  atoms  of  matter, 
and  carry  the  matter  to  which  they  cling  along  with 
them.  Every  body,  in  its  natural  condition,  pos- 
sesses both  fluids  in  equal  quantities.  As  long  as 
the  fluids  are  mixed  together  they  neutralise  each  other, 


What  is  Electricity  f  35 

the  body  in  which  they  are  thus  mixed  being  in  its 
natural  or  unelectrical  condition. 

By  friction  (and  by  various  other  means)  these  two 
fluids  may  be  torn  asunder,  the  one  clinging  by  pre- 
ference to  the  rubber,  the  other  to  the  body  rubbed. 

According  to  this  theory  there  must  always  be  at- 
traction between  the  rubber  and  the  body  rubbed,  be- 
cause, as  we  have  proved,  they  are  oppositely  electrified. 
This  is  in  fact  the  case.  And  mark  what  I  now  say. 
Over  and  above  the  common  friction,  this  electrical  at- 
traction has  to  be  overcome  whenever  we  rub  glass  with 
silk,  or  sealing-wax  with  flannel. 

You  are  too  young  to  fully  grasp  this  subject 
yet ;  and  indeed  it  would  lead  us  too  far  away  to 
enter  fully  into  it.  But  I  will  throw  out  for  future 
reflection  the  remark,  that  the  overcoming  of  the 
ordinary  friction  produces  heat  then  and  there  upon 
the  surfaces  rubbed,  while  the  force  expended  in  over- 
coming the  electric  attraction  may  be  converted  into 
heat  which  shall  appear  a  thousand  miles  away  from 
the  place  where  it  was  generated. 

Theoretic  conceptions  are  incessantly  checked  and 
corrected  by  the  advance  of  knowledge,  and  this  theory 
of  electric  fluids  is  doubted  by  many  eminent  scientific 
men.  It  will,  at  all  events,  have  to  be  translated  into 
a  form  which  shall  connect  it  with  heat  and  light, 
before  it  can  be  accepted  as  complete.  Nevertheless, 
keeping  ourselves  unpledged  to  the  theory,  we  shall 
find  it  of  exceeding  service  both  in  unravelling  ard  in 
connecting  together  electrical  phenomena. 


36  Lessons  in  Electricity. 


§   1 3.  Electric  Induction.     Definition  of  the  Term. 

We  have  now  to  apply  the  theory  of  electric  fluida 
to  the  important  subject  of  electric  induction. 

It  was  noticed  by  early  observers  that  contact  was 
not  necessary  to  electrical  excitement.  Otto  von 
Guericke,  as  we  have  already  seen  (§2),  found  that  a 
body  brought  near  his  sulphur  globe  became  electrical. 
By  bringing  his  excited  glass  tube  near  one  end  of  a 
conductor,  Stephen  Gray  attracted  light  bodies  at  the 
other  end.  He  also  obtained  attraction  through  the 
human  body.  From  the  human  body  also  Du  Fay, 
to  his  astonishment,  obtained  a  spark.  Canton,  in 
1753,  suspended  pith-balls  by  thread,  and  holding  an 
excited  glass  tube,  at  a  considerable  distance  from  them, 
caused  them  to  diverge.  On  removing  the  tube  the 
balls  fell  together,  no  permanent  charge  being  im- 
parted to  them.  Such  phenomena  were  further  studied 
and  developed  by  Wilcke  and  ^pinus,  Coulomb  anil 
Poisson. 

These  and  all  similar  results  are  embraced  by  the 
law,  that  when  an  electrified  body  is  brought  near  an 
unelectrified  conductor,  the  neutral  fluid  of  the  latter  is 
decomposed ;  one  of  its  constituents  being  attracted,  the 
other  repelled.  When  the  electrified  body  is  with- 
drawn, the  separated  electricities  flow  again  together 
and  render  the  conductor  unelectric. 

This  decomposition  of  the  neutral  fluid  by  the  mere 
presence  of  an  electrified  body  is  called  induction.  It 
is  also  called  electrification  by  influence. 

If,  while  it  is  under  the  influence  of  the  electrified 
body,  the  body  influenced  be  touched,  the  free  electri- 


Electric  Induction.     Definition  of  the  Term.    37 

city  (which  is  always  of  the  same  kind  as  that  of  the 
influencing  body)  passes  away,  the  opposite  electricity 
being  held  captive. 

On  removing  the  electrified  body  the  captive  elec- 
tricity is  set  free,  the  conductor  being  charger!  with 
electricity  opposite  in  kind  to  that  of  the  body  which 
electrified  it. 

You  cannot  do  better  here  than  repeat  Stephen 
Gray's  experiment.  Support  a  small  plank  or  lath,  L  if, 
fig.  14,  upon  a  warm  tumbler,  G,  and  bring  under  one  of 

Fio.  14. 


its  ends,  L,  and  within  four  or  five  inches  of  that  end, 
scraps  of  light  paper  or  of  gold  leaf.  Excite  your 
glass  tube,  E,  vigorously,  and  bring  it  over  the  other  end 
of  the  plank,  without  touching  it.  The  ends  may  be  six 
or  eight  feet  apart ;  the  light  bodies  will  be  attracted. 
The  experiment  is  easily  made,  and  you  are  not  to  rest, 
satisfied  till  you  can  make  it  with  ease  and  certainty. 

This  is  a  fit  place  to  repeat  that  you  must  keep  a 
close  eye  upon  the  tumblers  you  employ  for  insulation. 


58  Lessons  in  Electricity. 

Some  of  them,  made  of  common  glass,  are  hardly  to  be 
accounted  insulators  at  all. 


§  14.  Experimental  Researches  on  Electric  Induction. 

Our  mastery  over  this  subject  of  induction  must 
be  complete;  for  it  underlies  all  our  subsequent  enquiries. 
Without  reference  to  it  nothing  is  to  be   explained ; 
possessed  of  it  you  will  enjoy  not  only  a  wonderful 
power   of  explanation,   but   of  prediction.     We    will 
attack  it,  therefore,  with  the  determination  to  exhaust  it. 
And  here  a  slight  addition  must  be  made  to  our 
apparatus.     We  must  be  in  a  condition  to  take  samples 
FIG  15        °^  electricity,  and  to  convey  them,  with 
the  view  of  testing  them,  from  place  to 
place.     For  this  purpose  the  little  '  car- 
rier,'  shown    in   fig.    15,  will   be   found 
convenient.     T  is  a  bit  of  tin-foil,  two  or 
three   inches   square.     A  straw   stem   is 
stuck  on  to  it  by  sealing-wax,  the  lower 
end  of  the  stem  being  covered  by  seal- 
ing-wax.    To  make  the  insulation  sure, 
the  part  between  R  and  s'  is  wholly  of 
sealing-wax.      You   can   have   stems    of 
ebonite,  which  are    stronger,  for   a   few 
pence ;  but  you  can  have  this  one  for  a 
fraction  of  a  penny.     The  end  R'  is  to  be 
held  in  the  hand ;  the  electrified  body  is 
to  be  touched  by  T,  and  the  electricity 
conveyed  to  an  electroscope  to  be  tested. 

Touch  your  rubbed  glass  rod  with  T,  and  then  touch 
your  electroscope :  the  leaves  diverge  with  positive 
electricity.  Touch  your  rubbed  gutta-percha  or  seal- 


Experimental  Researches  on  Electric  Induction.  39 

ing-wax  with  T,  and  then  touch  your  electroscope :  the 
ieaves  diverge  with  negative  electricity.  If  the  elec- 
tricity of  any  body  augment  the  divergence  produced 
by  the  glass,  the  electricity  of  that  body  is  positive. 
If  it  augment  the  divergence  produced  by  the  gutta- 
percha,  the  electricity  is  negative.  And  now  we  are 
ready  for  further  work. 

Place  an  egg,  E,  fig.  1 6,  on  its  side  upon  a  dry  wine- 

FiQ.  16. 


glass  ;  bring  your  excited  glass  tube,  G,  within  an  inch  or 
so  of  the  end  of  the  egg.  What  is  the  condition  of  the 
egg?  Its  electricity  is  decomposed;  the  negative  fluid 
covering  the  end  a  adjacent  to  the  glass,  the  positive 
covering  the  other  end  6.  Kemove  the  glass  tube : 
what  occurs?  The  two  electricities  flow  together  and 
neutrality  restored.  Prove  this  neutrality.  Neither 
a  carrier  touching  the  egg,  nor  the  egg  itself,  has  any 
power  to  affect  your  electroscope,  or  to  attract  your 
balanced  lath. 

Again,  bring  the  excited  tube  near  the  egg.     Touch 
its   distant  part  b  with  your  carrier.     The  carrier  now 
4 


40  Lessons  in  Electricity. 

attracts  the  straw  (fig.  2)  or  the  balanced  lath  (fig.  4). 
It  also  causes  the  leaves  of  your  electroscope  to  diverge. 
What  is  the  quality  of  the  electricity  ?  It  repels  and  is 
repelled  by  rubbed  glass  ;  the  electricity  at  b  is  therefore 
positive.  Discharge  the  carrier  by  touching  it,  and 
bring  it  into  contact  with  the  end  a  of  the  egg  nearest 
to  the  glass  tube.  The  electricity  you  take  away  repels 
and  is  repelled  by  gutta-percha.  It  is  therefore 
negative.  Test  the  quality  also  by  the  electroscope. 

While  the  tube  G  is  near  the  egg  touch  the  end  6  with 
your  finger ;  now  try  to  charge  the  carrier  by  touching 
b  :  you  cannot  do  so — the  positive  electricity  has  dis- 
appeared. Has  the  negative  disappeared  also  ?  No. 
Kemove  the  glass  tube,  and  once  more  touch  the  egg 
at  b  by  the  carrier.  It  is  charged,  not  with  positive, 
but  with  negative  electricity.  Clearly  understand  this 
experiment.  The  neutral  electricity  of  the  egg  is  first 
decomposed  into  negative  and  positive ;  the  former 
attracted,  the  latter  repelled  by  the  excited  glass.  The 
repelled  electricity  is  free  to  escape,  and  it  has  escaped 
on  your  touching  the  egg  with  your  finger.  But  the 
attracted  electricity  cannot  escape  as  long  as  the  in- 
fluencing tube  is  held  near.  On  removing  the  tube 
which  holds  the  negative  fluid  in  bondage,  that  fluid 
immediately  diffuses  itself  over  the  whole  egg.  An 
apple,  or  a  turnip,  will  answer  for  these  experiments  at 
least  as  well  as  an  egg. 

Discharge  the  egg  by  touching  it.  .Re-excite  the 
glass  tube  and  bring  it  again  near.  Touch  the  egg 
with  a  wire  or  with  your  finger  at  a.  Is  it  the  negative  at 
a,  into  which  you  plunge  your  finger,  that  escapes  ?  No 
such  thing.  The  free  positive  fluid  passes  through  the 
negative,  and  through  your  finger  to  the  earth.  Prove 


Experimental  Researches  on  Electric  Induction.  41 

this  by  removing,  first,  your  finger,  and  then  the  glass 
tube.     The  egg  is  charged  negatively. 

Again  ;  place  two  eggs,  E  E,  fig.  17,  lengthwise  on  two 
dry  wine-glasses,  g  g,  and  cause  two  of  their  ends  to 

FIG.  17. 


touch  each  other,  as  at  c.  Bring  your  rubbed  glass  rod 
near  the  end  a,  and  while  it  is  there  separate  the  eggs  by 
moving  one  glass  away  from  the  other.  Withdraw  the 
rod  and  test  both  eggs,  a  repels  rubbed  sealing-wax,  and 
6  repels  rubbed  glass  ;  a  is  therefore  negative,  b  is  posi- 
tive. The  two  charges,  moreover,  exactly  neutralise  each 
other  in  the  electroscope.  Again  bring  the  eggs  toge- 
ther and  restore  the  rubbed  tube  to  its  place  near  a. 
Touch  a  and  then  separate  the  eggs.  Eemove  the  glass 
rod  and  test  the  eggs,  a  is  negative,  6  is  neutral.  Its 
electricity  has  escaped  through  the  finger,  though  placed 
at  a. 

Equally  good,  if  not  indeed  more  handy,  for  these 
experiments  are  two  apples  A  A,  fig.  18,  supported  on 
stems  of  sealing-wax.  A  needle  is  heated  and  sunk  in 
each  case  into  the  stick  of  wax  at  the  top,  and  on  to  the 
needle  the  apple  is  pushed.  The  sealing-wax  stems  are 


42 


Lessons  in  Electricity. 


ptuck  on  by  melting  to  little  foot-boards.     By  arrange- 
ments of  this  kind  you  make  experiments  which  are 


Fm.  18. 


more  instructive  than  those  usually  made  with  instiu- 
ments  twenty  times  more  expensive. 


Fio.  19. 


r i 

Push  your  researches  still  farther,  and   instead  of 
-'n^ing  the  eggs  or  apples  together,  place  them  six  feet 


Experimental  Researches  on  Electric  Induction.  43 

or  so  apart,  and  let  a  light  chain,  c,  fig.  1 9,  or  a  wire, 
stretch  from  one  to  the  other.  Two  brass  balls,  or  wooden 
balls  covered  with  tin-foil,  supported  by  tall  drinking 
glasses,  G  G',  will  be  better  than  the  eggs  for  this  experi- 
ment, for  they  will  bear  better  the  strain  of  the  chain  ; 
but  you  can  make  the  experiment  with  the  eggs,  or  very 
readily  with  the  two  apples  or  two  turnips.  For  the  pre- 
sent we  will  suppose  the  straw-index  1 i'  not  to  be  ther^. 
Eub  your  glass  tube  R,  and  bring  it  near  one  of  the 
balls ;  test  both:  the  near  one,  T7,  is  negative,  the  distant 
one,  T,  positive.  Touch  the  near  one,  the  positive  elec- 
tricity, which  had  been  driven  along  the  chain  to  the 
remotest  part  of  the  system,  returns  along  the  chain, 
passes  through  the  negative,  which  is  held  captive  by 
the  tube,  and  escapes  to  the  earth.  When  the  tube  R 
is  removed,  negative  electricity  overspreads  both  chain 
and  balls. 

In  fig.  8  you  made  the  acquaintance  of  the  plate  N, 
and  the  straw-index  1 i',  shown  on  a  smaller  scale  in 
fig.  19.  By  their  means  you  immediately  see  both  the 
effect  of  the  first  induction,  and  the  consequence  of 
touching  any  part  of  the  system  with  the  finger.  The 
plate  N  rests  over  the  ball  or  turnip  T,  the  position  of 
the  straw-index  being  that  shown  by  the  dots.  Bring 
the  rubbed  tube  near  T? :  the  end  N  of  the  index  im- 
mediately descends  and  the  other  end  rises  along  the 
graduated  scale.  Remove  the  glass  rod ;  the  index  I  i' 
immediately  falls.  Practise  this  approach  and  with- 
drawal, and  observe  how  promptly  the  index  declares 
the  separation  and  recomposition  of  the  fluids. 

While  the  tube  is  near  T',  and  the  end  N  of  the  index 
is  attracted,  let  if  be  touched  by  the  finger.  The  end  N 
is  immediately  liberated,  for  the  electricity  which  pulled 


44 


Lessons  in  Electricity. 


ft  down  escapes  along  the  chain  and  through  the  finger 
to  the  earth.  Now  remove  your  excited  tube.  The 
captive  negative  electricity  diffuses  itself  over  both  balls, 
and  the  index  is  again  attracted. 

Instead  of  the  chain  you  may  interpose  between  the 
balls  100  feet  of  wire  supported  by  silk  loops.  This  is 
done  in  fig.  20,  which  shows  the  wire  w  supported  by 

FIG.  20. 


the  silk  strings  s  s  s.  For  the  ball  or  turnip  i7,  fig. 
1 9,  the  cylinder  c,  on  a  glass  support  G,  is  substituted, 
the  little  table  M  taking  the  place  of  the  ball  T.  Every 
approach  and  withdrawal  of  the  rubbed  glass  tube  R  is 
followed  obediently  by  the  attraction  and  liberation  of 
N,  and  the  corresponding  motion  of  the  index  N  I. 

Kepeat  here  an  experiment,  first  made  by  a  great  elec- 
trician named  ^Epinus.  I  wish  you  to  make  these 
historic  experiments.  Insulate  an  elongated  metal  con- 
ductor, c  c',  fig.  21,  or  one  formed  of  wood  coated  with 
tin-foil — even  a  carrot,  cucumber,  or  parsnip,  so  that 


Experimental  Researches  on  Electric  Induction.  45 

it  be  insulated,  will  answer.  Let  a  small  weight,  w, 
suspended  from  a  silk  string,  s,  rest  on  one  end  of  the 
conductor,  and  hold  your  rubbed  glass  tube,  K,  over  the 


FIG.  21. 


other  end.  You  can  predict  beforehand  what  will  occur 
when  you  remove  the  weight.  It  carries  away  with  it 
electricity,  which  repels  rubbed  glass,  and  attracts  your 
balanced  lath. 

Stand  on  an  insulating  stool ;  or  make  one  by 
placing  a  board  on  four  warm  tumblers.  Present 
the  knuckles  of  your  right  hand  to  the  end  of  the 
balanced  lath,  and  stretch  forth  your  left  arm.  There 


It) 


Lessons  in  Electricity. 


is  no  attraction.  But  let  a  friend  or  an  assistant 
bring  the  rubbed  glass  tube  over  the  left  arm ;  the 
lath  immediately  follows  the  right  hand. 

Touch  the  lath,  or  any  other  uninsulated  body ;  the 
'  attractive  virtue,'  as  it  was  called  by  Gray,  disappears. 
After  this,  as  long  as  the  excited  tube  is  held  over  the 
arm  there  is  no  attraction.  But  when  the  tube  is 
removed  the  attractive  power  of  the  hand  is  restored. 
Here  the  first  attraction  was  by  positive  electricity 
driven  to  the  right  hand  from  the  left,  and  the  second 
attraction  by  negative  electricity,  liberated  by  the  re- 
moval of  the  glass  rod.  Experiment  proves  the  logic 
of  theory  to  be  without  a  flaw. 

Stand  on  an  insulating  stool,  and  place  your  right 
hand  on  the  electroscope  :  there  is  no  action.  Stretch 
forth  the  left  arm  and  permit  an  assistant  alternately 
to  bring  near,  and  to  withdraw,  an  excited  glass  tube. 
The  gold  leaves  open  and  collapse  in  similar  alternation. 
At  every  approach,  positive  electricity  is  driven  over  the 
gold  leaves ;  at  every  withdrawal,  the  equilibrium  is 
restored. 

We  are  now  in  a  condition  to  repeat,  with  ease,  the 
FIG.  22. 


experiment  of  Du  Fay  mentioned  in  §  13.     A  board  is 
supported   by  four   silk   ropes,  and    on   the   board  i? 


Experimental  Researches  on  Electric  Induction.  47 

stretched  a  boy.  Bring  his  forehead,  or  better  still  his 
nose,  under  the  end  of  your  straw  index  1 i7,  fig.  22. 
Then  bring  down  over  his  legs  your  rubbed  glass  tube ; 
instantly  the  end  i'  is  attracted  and  the  end  I  rises 
along  the  graduated  scale.  Before  the  end  i'  comes 
into  contact  witli  the  nose  or  forehead  a  spark  passes 
between  it  and  the  boy. 

I  will  now  ask  you  to  charge  your  Dutch  metal  elec- 
troscope (fig.  7)  positively  by  rubbed  gutta-percha,  and 
to  charge  it  negatively  by  rubbed  glass.  A  moment's 
reflection  will  enable  you  to  do  it.  You  bring  your 
excited  body  near  :  the  same  electricity  as  that  of  the 
excited  body  is  driven  over  the  leaves,  and  they  diverge 
by  repulsion.  Touch  the  electroscope,  the  leaves  col- 
lapse. Withdraw  your  finger,  and  withdraw  afterwards 
the  excited  body :  the  leaves  then  diverge  with  the 
opposite  electricity. 

The  simplest  way  of  testing  the  quality  of  electri- 
city is  to  charge  the  electroscope  with  electricity  of  a 
known  kind.  If,  on  the  approach  of  the  body  to  be 
tested,  the  leaves  diverge  still  wider,  the  leaves  and  the 
body  are  similarly  electrified.  The  reason  is  obvious. 

Omitting  the  last  experiment,  the  wealth  of  know- 
ledge which  these  researches  involve  might  be  placed 
within  any  intelligent  boy's  reach  by  the  wise  expen- 
diture of  half-a~crown. 

Once  firmly  possessed  of  the  principle  of  induction 
and  versed  in  its  application,  the  difficulties  of  our  subject 
will  melt  away  before  us.  In  fact  our  subsequent  work 
will  consist  mainly  in  unravelling  phenomena  by  the 
aid  of  this  principle. 

Without  a   knowledge  of  this  principle  we  could 


48  Lessons  in  Electricity. 

render  no  account  of  the  attraction  of  neutral  bodies 
by  our  excited  tubes.  In  reality  the  attracted  bodies 
are  not  neutral :  they  are  first  electrified  by  influence, 
and  it  is  because  they  are  thus  electrified  that  they 
are  attracted. 

This  is  the  place  to  refer  more  fully  to  a  point 
already  alluded  to.  Neutral  bodies,  as  just  stated, 
are  attracted,  because  they  are  really  converted  into 
electrified  bodies  by  induction.  Suppose  a  body  to  be 
feebly  electrified  positively,  and  that  you  bring  your 
rubbed  glass  tube  to  bear  upon  the  body.  You  clearly 
see  that  the  induced  negative  electricity  may  be  strong 
enough  to  mask  and  overcome  the  weak  positive  charge 
possessed  by  the  body.  We  should  thus  have  two  bodies 
electrified  alike,  attracting  each  other.  This  is  the 
danger  against  which  I  promised  to  warn  you  in  §  10, 
where  the  test  of  attraction  was  rejected. 

We  will  now  apply  the  principle  of  induction  to 
explain  a  very  beautiful  invention,  made  known  by 
the  celebrated  Volta  in  1775. 

§  15.  The  Electrophorus. 

Cut  a  circle,  T,  fig.  23,  6  inches  in  diameter  out  of 
sheet  zinc,  or  out  of  common  tin.  Heat  it  at  its  centre 
by  the  flame  of  a  spirit-lamp  or  of  a  candle.  Attach  to 
it  there  a  stick  of  sealing-wax,  H,  which,  when  the  metal 
cools,  is  to  serve  as  an  insulating  handle. — You  have  now 
the  lid  of  the  electrophorus.  A  resinous  surface,  or  what 
is  simpler  a  sheet  of  vulcanised  india-rubber,  p,  or  even 
of  hot  brown  paper,  will  answer  fo :  the  plate  of  the 
electrophorus. 

Rub  your  'plate '  with  flannel,  or  whisk  it  briskly 


The  Electrophorus. 


49 


with  a  fox's  brush.     It  is  thereby  negatively  electrified. 
Place  the  « lid '  of  your '  electrophorus  on  the  excited 


FIG.  23. 


surface :  it  touches  it  at  a  few  points  only.  For  the 
most  part  lid  and  plate  are  separated  by  a  film  of 
air. 

The  excited  surface  acts  by  induction  across  this 
film  upon  the  lid,  attracting  its  positive  and  repelling 
its  negative  electricity.  You  have  in  fact  in  the  lid 
two  layers  of  electricity,  the  lower  one,  which  is  'bound,' 
positive  ;  the  upper  one,  which  is  '  free,'  negative.  Lift 
the  lid  :  the  electricities  flow  again  together  ;  neutrality 
is  restored,  and  your  lid  fails  to  attract  your  balanced 
lath. 

Once  more  place  the  lid  upon  the  excited  surface : 
touch  it  with  the  finger.  What  occurs  ?  You  ought 
to  know.  The  free  electricity,  which  is  negative,  will 
escape  through  your  body  to  the  earth,  leaving  the 
chained  positive  behind. 

Now  lift  the  lid  by  the  handle :  what  is  its  condi- 
tion ?  Again  I  say  you  ought  to  know.  It  is  covered 


50  Lessons  in  Electricity. 

with  free  positive  electricity.  If  it  be  presented  to  the 
lath  it  will  strongly  attract  it:  if  it  be  presented  to  the 
knuckle  it  will  yield  a  spark. 

A  smooth  half-crown,  or  a  penny,  will  answer  for  this 
experiment.  Stick  to  the  coin  an  inch  of  sealing-wax 
as  an  insulating  handle  :  bring  it  down  upon  the  excited 
india-rubber :  touch  it,  lift  it,  and  present  it  to  your 
lath.  The  lath  may  be  six  or  eight  feet  long,  three  inches 
wide  and  half  an  inch  thick ;  the  little  electrophorus  lid, 
formed  by  the  half  crown,  will  pull  it  round  and  round. 
The  experiment  is  a  very  impressive  one. 

Scrutinise  your  instrument  still  further.  Let  the 
end  of  a  thin  wire  rest  upon  the  lid  of  your  electro- 
phorus, under  a  little  weight  if  necessary ;  and  connect 
the  other  end  of  the  wire  with  the  electroscope.  As 
you  lower  the  lid  down  towards  the  excited  plate  of  the 
electrophorus,  what  must  occur  ?  The  power  of  previ- 
sion now  belongs  to  you  and  you  must  exercise  it.  The 
repelled  electricity  will  flow  over  the  leaves  of  the 
electroscope,  causing  them  to  diverge.  Lift  the  lid, 
they  collapse.  Lower  and  raise  the  lid  several  times, 
and  observe  the  corresponding  rhythmic  action  of  the 
electroscope  leaves. 

A  little  knob  of  sealing-wax,  B,  coated  with  tin-foil, 
or  indeed  any  knob  with  a  conducting  surface,  stuck  to 
the  lid  of  the  electrophorus,  will  enable  you  to  obtain  a 
better  spark.  The  reason  of  this  will  Immediately 
appear. 

More  than  half  the  value  of  your  present  labour 
consists  in  arranging  each  experiment  in  thought  before 
it  is  realised  in  fact ;  and  more  than  half  the  delight  of 
your  work  will  consist  in  observing  the  verification  of 
what  you  have  foreseen  and  predicted. 


Action  of  Points  and  Flames.  61 


§  16.  Action  of  Points  and  Flames. 

The  course  of  exposition  proceeds  naturally  from 
the  electrophorus  to  the  electrical  machine.  But  be- 
fore we  take  up  the  machine  we  must  make  our  minds 
clear  regarding  the  manner  in  which  electricity  diffuses 
itself  over  conductors,  and  more  especially  over  elongated 
and  pointed  conductors. 

Kub  your  glass  tube  and  draw  it  over  an  insulated 
sphere  of  metal — of  wood  covered  with  tin-foil,  or  indeed 
any  other  insulated  spherical  conductor.  Eepeat  the 
process  several  times,  so  as  to  impart  a  good  charge  to 
the  sphere.  Touch  the  charged  sphere  with  your 
carrier,  and  transfer  the  charge  to  the  electroscope. 
Note  the  divergence  of  the  leaves.  Discharge  the 
electroscope,  and  repeat  the  experiment,  touching,  how- 
ever, some  other  point  of  the  sphere.  The  electroscope 
shows  sensibly  the  same  amount  of  divergence.  Even 
when  the  greatest  exactness  of  the  most  practised  ex- 
perimenter is  brought  into  play,  the  spherical  conductor 
is  found  to  be  equally  charged  at  all  points  of  its 
surface.  You  may  figure  the  electric  fluid  as  a  little 
ocean  encompassing  the  sphere,  and  of  the  same  depth 
everywhere. 

But  supposing  the  conductor,  instead  of  being  a 
sphere,  to  be  a  cube,  an  elongated  cylinder,  a  .cone,  or 
a  disk.  The  depth,  or  as  it  is  sometimes  called  the 
density  of  the  electricity,  will  not  be  everywhere  the 
game.  The  corners  of  the  cube  will  impart  a  stronger 
charge  to  your  carrier  than  the  sides.  The  end  of  the 
cylinder  will  impart  a  stronger  charge  than  its  middle. 
The  edge  of  the  disk  will  impart  a  stronger  charge  than 


52 


Lessons  in  Electricity. 


its  flat  surface.  The  apex  or  point  of  the  cone  will 
impart  a  stronger  charge  than  its  curved  surface  or  its 
base. 

You  can  satisfy  yourself  of  the  truth  of  all  this  in  a 
rough,  but  certain  way,  by  charging,  after  the  sphere,  a 
turnip  cut  into  the  form  of  a  cube;  or  a  cigar-box 
coated  with  tin-foil  ;  a  metal  cylinder,  or  a  wooden  one 
coated  with  tin-foil  ;  a  disk  of  tin  or  of  sheet  zinc  ; 
a  carrot  or  parsnip  with  its  natural  shape  improved 
so  as  to  make  it  a  sharp  cone.  You  will  find  the 
charge  imparted  to  the  carrier  by  the  sharp  corners 
and  points  of  such  bodies,  when  electrified,  to  be  greater 
than  that  communicated  by  the  gently  rounded  or  flat 
surfaces.  The  difference  may  not  be  great,  but  it  will 
be  distinct.  Indeed  an  egg  laid  on  its  side,  as  we  have 

FIG.  24. 


already  used  it  in  our  experiments  on  induction  (fig. 
16),  yields  a  stronger  charge  from  its  ends  than  from 
its  middle. 

Let  me  place  before  you  an  example  of  this  distribu- 
tion, taken  from  the  excellent  work  on  '  Frictional  Elec- 
tricity '  by  Professor  Eiess  of  Berlin.  Two  cones,  fig. 
24,  are  placed  together  base  to  base.  Calling  the  strength 
of  the  charge  along  the  circular  edge  where  the  two 
bases  join  each  other  100,  the  charge  at  the  apex  of  the 
blunter  cone  is  133;  and  at  the  apex  of  the  sharper  one 


Action  of  Points  and  Flames.  53 

202.     The  other  numbers  give  the  charges  taken  from 
the   points  where  they  are  placed.     Fig.  25,  moreover, 


represents  a  cube  with  a  cone  placed  upon  it.  The 
charge  on  the  face  of  the  cube  being  1,  the  charges  at 
the  corners  of  the  cube  and  at  the  apex  of  the  cone 
are  given  by  the  other  numbers  ;  they  are  all  far  in  ex- 
cess of  the  electricity  on  the  flat  surface. 

Eiess  found  that  he  could  deduce  with  great 
accuracy  the  sharpness  of  a  point,  from  the  charge 
whicli  it  imparted.  He  compared  in  this  way  the 
sharpness  of  various  thorns,  with  that  of  a  fine  English 
sewing  needle.  The  following  is  the  result : — Euphor- 
bia thorn  was  sharper  than  the  needle ;  gooseberry 
thorn  of  the  same  sharpness  as  the  needle ;  while  cactus, 
blackthorn,  and  rose,  fell  more  and  more  behind  the 
needle  in  sharpness.  Calling,  for  example,  the  charge 
obtained  from  euphorbia  90 ;  that  obtained  from  the 
needle  was  80,  and  from  the  rose  only  53. 

Considering  that  each  electricity  is  self-repulsive,  and 
that  it  heaps  itself  up  upon  a  point  in  the  manner  here 
shown,  you  will  have  little  difficulty  in  conceiving  that 


54  Lessons  in  Electricity. 

when  the  charge  of  a  conductor  carrying  a  point  is 
sufficiently  strong,  the  electricity  will  finally  disperse 
itself  by  streaming  from  the  point. 

The  following  experiments  are  theoretically  impor- 
tant : — Attach  a  stick  of  sealing-wax  to  a  small  plate 
of  tin  or  of  wood,  so  that  the  stick  may  stand  upright. 
Heat  a  needle  and  insert  it  into  the  top  of  the  stick 
of  wax;  on  this  needle  mount  horizontally  a  carrot. 
You  have  thus  an  insulated  conductor.  Stick  into  your 
carrot  at  one  of  its  ends  a  sewing  needle;  and  hold  for 
an  instant  your  rubbed  glass  tube  in  front  of  this  needle 
without  touching  it.  What  occurs  ?  The  negative 
electricity  of  the  carrot  is  immediately  discharged  from 
the  point  against  the  glass  tube.  Eemove  the  tube,  test 
the  carrot :  it  is  positively  electrified. 

And  now  (or  another  experiment,  not  so  easily  made, 
but  still  certain  to  succeed  if  you  are  careful.  Excite 
your  glass  rod,  turn  your  needle  away  from  it,  and  bring 
the  rod  near  the  other  end  of  the  carrot.  What  occurs? 
The  positive  electricity  is  now  repelled  to  the  point, 
from  which  it  will  stream  into  the  air.  Eemove  the 
rod  and  test  the  carrot :  it  is  negatively  electrified. 

Again  turn  the  point  towards  you,  and  place  in 
front  of  it  a  plate  of  dry  glass,  wax,  resin,  shellac, 
paraffin,  gutta-percha,  or  any  ether  insulator.  Pass  your 
rubbed  glass  tube  once  downwards  or  upwards,  the  in- 
sulating plate  being  between  the  excited  tube  and  the 
point.  The  point  will  discharge  its  electricity  against 
tbe  insulating  plate,  vrhich  on  trial  will  be  found  nega- 
tively electrified. 


The  Electrical  Machine. 


55 


§  17.  The  Electrical  Machine. 

An  electrical  machine  consists  of  two  principal  parts; 
the  insulator  which  is  excited  by  friction,  and  the 
*  prime  conductor.' 

The  sulphur  sphere  of  Otto  von  Guericke  was,  as 
already  stated,  the  first  electrical  machine.  The  hand 
was  the  rubber,  and  indeed  it  long  continued  to  be  so. 
For  the  sulphur  sphere,  Hauksbee  and  Winckler  substi- 
tuted globes  of  glass.  Boze  of  Wittenberg  (1741) 
added  the  prime  conductor,  which  was  at  first  a  tin 
tube  supported  by  resin,  or  suspended  by  silk.  Soon 
afterwards  Gordon  substituted  a  glass  cylinder  for 
the  globe.  It  was  sometimes  mounted  vertically, 
sometimes  horizontally.  Gordon  so  intensified  his 
discharges  as  to  be  able  to  kill  small  birds  with 
them.  In  1760  Planta  introduced  the  plate  machine 

now  commonly  in  use. 

FIG.  26. 


Mr.  Cottrell  has  constructed  for  these  Lessons  the 
small  cylinder  machine  shown  in  fig.  26.     The  glass 
5 


56  Lessons  in  Electricity. 

cylinder  is  about  7  inches  long  and  4  inches  in  dia- 
meter: its  cost  is  eighteen  pence.  Through  the 
cylinder  passes  tightly,  as  an  axis,  a  piece  of  lath, 
rendered  secure  by  sealing-wax  where  it  enters  and  where 
it  quits  the  cylinder.  G  is  a  glass  rod  supporting  the 
conductor  c,  which  is  a  piece  of  lath  coated  with  tin- 
foil. Into  the  lath  is  driven  the  series  of  pin  points, 
p,  P.  The  rubber  K,  is  seen  at  the  further  side  of  the 
cylinder,  supported  by  the  upright  lath  R',  and  caused 
to  press  against  the  glass,  s'  is  a  flap  of  silk  attached 
to  the  rubber.  When  the  handle  is  turned  sparks  may 
be  taken,  or  a  Leyden  jar  *  charged  at  the  knob  c. 

A  plate  machine  is  shown  in  fig.  27.     p  is  the  plate, 
which  turns  on  an  axis    passing  through  its  centre : 

FIG.  27. 


B  and  a'  are  two  rubbers  which  clasp  the  plate,  with  the 
flaps  of  silk  s  s*  attached  to  them.     A  and  A'  are  rows  of 

1  To  be  subsequently  explained. 


.  The  Electrical  Machine.  57 

points  forming  part  of  the  prime  conductor,  c.  G  G'  is  an 
insulating  rod  of  glass,  which  cuts  off  the  connection 
between  the  conductor  and  the  handle  of  the  machine. 

The  prime  conductor  is  charged  in  the  following 
manner.  When  the  glass  plate  is  turned,  as  it  passes 
each  rubber  it  is  positively  electrified.  Facing  the 
electrified  glass  is  the  row  of  points,  placed  midway 
between  the  two  rubbers.  On  these  points  the  glass  acts 
by  induction,  attracting  the  negative  and  repelling 
the  positive.  In  accordance  with  the  principles 
already  explained  in  §  16,  the  negative  electricity 
streams  from  the  points  against  the  excited  glass,  which 
then  passes  on  neutralised  to  the  next  rubber,  where  it 
is  again  excited. 

Thus  the  prime  conductor  is  charged,  not  by  the 
direct  communication  to  it  of  positive  electricity,  but 
by  depriving  it  of  its  negative. 

If  when  the  conductor  is  charged  you  bring  the 
knuckle  near  it,  the  electricity  passes  from  the  con- 
ductor to  the  knuckle  in  the  form  of  a  spark. 

Take  this  spark  with  the  blunt  knuckle  while  the 
machine  is  being  turned;  and  then  try  the  effect  of 
presenting  the  finger  ends,  instead  of  the  knuckle,  to 
the  conductor.  The  spark  falls  exceedingly  in  bril- 
liancy. Substitute  for  the  finger  ends  a  needle  point : 
you  fail  to  get  a  spark  at  all.  To  obtain  a  good  spark 
the  electricity  upon  the  prime  conductor  must  reach 
a  sufficient  density  (or  tension  as  it  is  sometimes 
called) ;  and  to  secure  this  no  points  from  which  the 
electricity  can  stream  out  must  exist  on  the  conductor, 
or  be  presented  to  it.  All  parts  of  the  conductor  are 
therefore  carefully  rounded  off.  sharp  points  and  edges 
being  avoided. 


58  Lessons  in  Electricity. 

It  is  usual  to  attach  to  the  conductor  an  electro- 
scope consisting  of  an  upright  metal    stern,  A  c,  fig, 
FIG.  28.  28,  to  which  a  straw  with  a  pitli 

ball,  B,  at  its  free  end,  is  attached. 
The  straw  turns  loosely  upon  a 
pivot  at  c.  The  electricity  pass- 
ing from  the  conductor  is  diffused 
over  the  whole  electroscope,  and 
the  straw  and  stem  being  both 
positively  electrified,  repel  each 
other.  The  straw,  being  the 
movable  body,  flies  away.  The 
amount  of  the  divergence  is 
measured  upon  a  graduated  arc. 

§  18.  Further  Experiments  on  the  Action  of  Points. 
The  Electric  Mill.  The  Gulden  Fish.  Lightning 
Conductors. 

If  no  point  exist  on  the  conductor,  a  single  turn  of 
the  handle  of  the  machine  usually  suffices  to  cause  the 
straw  to  stand  out  at  a  large  angle  to  the  stem.  If,  on 
the  contrary,  a  point  be  attached  to  the  conductor, 
you  cannot  produce  a  large  divergence,  because  the 
electricity,  as  fast  as  it  is  generated,  is  dispersed  by 
the  point.  The  same  effect  is  observed  when  you  pre- 
sent a  point  to  the  conductor.  The  conductor  acts  by 
induction  upon  the  point,  causing  the  negative  electri- 
city to  stream  from  it  against  the  conductor,  which  is 
thus  neutralised  almost  as  fast  as  it  is  charged.  Flames 
and  glowing  embers  act  like  points ;  they  also  rapidly 
discharge  electricity. 

The  electricity  escaping  from  a  point  or  flame  into 
the  air  renders  the  air  self-repulsive.  The  consequence 


The  Electric  Mill 


59 


is  that  when  the  hand  is  placed  over  a  point  mounted 
on  the  prime  conductor  of  a  machine  in  good  action,  a 
cold  blast  is  distinctly  felt.  Dr.  Watson  noticed  this 
blast  from  a  flame  placed  on  an  electrified  conductor ; 
while  Wilson  noticed  the  blast  from  a  point.  Jallabert 
and  the  Abbe  Nollet  also  observed  and  described  the 
influence  of  points  and  flames.  The  blast  is  called  the 
'  electric  wind.'  Wilson  moved  bodies  by  its  action : 
Faraday  caused  it  to  depress  the  surface  of  a  liquid : 
Hamilton  employed  the  reaction  of  the  electric  wind 
to  make  pointed  wires  rotate.  The  '  wind '  was  also 
found  to  promote  evaporation. 

Hamilton's  apparatus  is  called  the  c  electric  mill.' 
Make  one  for  yourself  thus  :  Place  two  straws  s  s,  s'  /, 

FIG.  29. 


fig.  29,  about  eight  inches  long,  across  each  other  at  a 
right  angle.  Stick  them  together  at  their  centres  by  a 
bit  of  sealing-wax.  Pass  a  fine  wire  through  each  straw 
and  bend  it  where  it  issues  from  the  straw,  so  as  to 


60  Lessons  in  Electricity. 

form  a  little  pointed  arm  perpendicular  to  the  straw,  and 
from  half  an  inch  to  three-quarters  of  an  inch  long.  It 
is  easy,  by  means  of  a  bit  of  cork  or  sealing-wax,  to 
fix  the  wire  so  that  the  little  bent  arms  shall  point 
not  upwards  or  downwards,  but  sideways,  when  the  cross 
is  horizontal.  The  points  of  sewing  needles  may  also 
be  employed  for  the  bent  arms.  A  little  bit  of  straw 
stuck  into  the  cross  at  the  centre,  forms  a  cap.  This 
slips  over  a  sewing  needle,  N,  supported  by  a  stick  of  seal- 
ing-wax, A.  Connect  the  sewing  needle  with  the  electric 
machine,  and  turn.  A  wind  of  a  certain  force  is  dis- 
charged from  every  point,  and  the  cross  is  urged  round 
with  the  same  force  in  the  opposite  direction. 

You  might  easily,  of  course,  so  arrange  the  points 
that  the  wiad  from  some  of  them  would  neutralise  the 
wind  from  others.  But  the  little  pointed  arms  are  to  be 
so  bent  that  the  reaction  in  every  case  shall  not  oppose 
but  add  itself  to  the  others. 

The  following  experiments  will  yield  you  important 
information  regarding  the  action  of  points.  Stand,  as 
you  have  so  often  done  before,  upon  a  board  supported 
by  four  warm  tumblers.  Hold  a  small  sewing  needle, 
with  its  point  defended  by  the  fore  finger  of  your  right 
hand,  towards  your  Dutch  metal  electroscope.  Place 
your  left  hand  on  the  prime  conductor  of  your  machine. 
Let  the  handle  be  turned  by  a  friend  or  an  assistant : 
the  leaves  of  the  electroscope  open  out  a  little.  Un- 
cover the  needle  point  by  the  removal  of  your  finger ; 
the  leaves  at  once  fly  violently  apart. 

Mount  a  stout  wire  upright  on  the  conductor,  c,  fig. 
30,  of  your  machine ;  or  support  the  wire  by  sealing- 
wax,  gutta-percha  or  glass,  at  a  distance  from  the 


Experiments  on  Action  of  Points. 


61 


conductor,  and  connect  both  by  a  fine  wire.     Bend  your 
stout  wire  into  a  hook,  and  hang  from  it  a  tassel,  T, 


Fio.  30. 


composed  of  many  strips  of  light  tissue  paper.  Work 
the  machine.  Electricity  from  the  conductor  flows 
over  the  tassel,  and  the  strips  diverge.1  Hold  your 
closed  fist  towards  the  tassel,  the  strips  of  paper 
stretch  towards  it.  Hold  the  needle,  defended  by  the 
finger,  towards  the  tassel :  attraction  also  ensues. 
Uncover  the  needle  without  moving  the  hand;  the 
strips  retreat  as  if  blown  away  by  a  wind.  Holding  the 
needle  N,  fig.  31,  upright  underneath  the  tassel,  its 
strips  discharge  themselves  and  collapse  utterly. 

And  now  repeat  Du  Fay's  experiment  which  led  to 

1  This  is  always  the  case  in  London.     Still  even  here  some  daya 
wo  so  dry  as  to  render  it  difficult  to  electrify  the  tassel. 


6S 


Lessons  in  Electricity. 

FIG.  31. 


the  discovery  of  two  electricities.  Excite  your  glass 
tube,  and  hold  it  in  readiness  while  a  friend,  or  an 
assistant,  liberates  a  real 
gold  or  silver  leaf  in  the 
air.  Bring  the  tube  near 
the  leaf:  it  plunges  to- 
.  wards  the  tube,  stops 
suddenly,  and  then  flies 
away.  You  may  chase 
it  round  the  room  for 
hours  without  permitting 
it  to  reach  the  ground. 
The  leaf  is  first  acted 
upon  inductively  by  the 
tube.  It  is  powerfully 
attracted  for  a  moment, 
and  rushes  towards  the 
tube.  But  from  its  thin 


Experiments  on  Action  of  Points. 


68 


edges  and  corners  the  negative  electricity  streams  forth, 
leaving  the  leaf  positively  electrified.  Repulsion  then 
sets  in,  because  tube  and  leaf  are  electrified  alike,  as 
shown  in  fig.  32.  The  retreat  of  the  tassel  in  the  last 
experiment  is  due  to  a  similar  cause. 

There  is  also   a   discharge   of  positive   electricity 
into  the   air   from  the  more  distant    portions  of  the 

FIG.  33. 


gold-leaf,  to  which  that  electricity  is  repelled.  Both 
discharges  are  accompanied  by  an  electric  wind.  It  ia 
possible  to  give  the  gold-leaf  a  shape  which  shall  enable 
it  to  float  securely  in  the  air,  by  the  reaction  of  the  two 
winds  issuing  from  its  opposite  ends.  This  is  Franklin's 
experiment  of  the  Golden  Fish.  It  was  first  made  with 
the  charged  conductor  of  an  electrical  machine.  M. 


64  Lessons  in  Electricity. 

Srtsczek  revived  it  in  a  more  convenient  form,  using 
instead  of  the  conductor  the  knob  of  a  charged  Leyden 
jar.  You  may  walk  round  a  room  with  the  jar  in  your 
hand  ;  the  '  fish '  will  obediently  follow  in  the  air  an 
inch  or  two,  or  even  three  inches,  from  the  knob.  See 
A  B,  fig.  33.  Even  a  hasty  motion  of  the  jar  will  not 
shake  it  away. 

Well-pointed  lightning  conductors,  when  acted  on 
by  a  thunder  cloud,  discharge  their  induced  electricity 
against  the  cloud.  Franklin  saw  this  with  great  clear- 
ness, and  illustrated  it  with  great  ingenuity.  The 
under  side  of  a  thunder  cloud,  when  viewed  horizon- 
tally, he  observed  to  be  ragged,  composed,  in  fact,  of 
fragments  one  below  the  other,  sometimes  reaching 
near  the  earth.  These  he  regarded  as  so  many 
stepping-stones  which  assist  in  conducting  the  stroke 
of  the  cloud.  To  represent  these  by  experiment  he 
took  two  or  three  locks  of  fine  loose  cotton,  tied  them 
in  a  row,  and  hung  them  from  his  prime  conductor. 
When  this  was  excited  the  locks  stretched  downwards 
towards  the  earth  ;  but  by  presenting  a  sharp  point 
erect  under  the  lowest  bunch  of  cotton,  it  shrunk  up- 
wards to  that  above  it,  nor  did  the  shrinking  cease  till 
all  the  locks  had  retreated  to  the  prime  conductor 
itself.  '  May  not,'  says  Franklin,  '  the  small  electrified 
clouds,  whose  equilibrium  with  the  earth  Is  so  soon 
restored  by  the  point,  rise  up  to  the  main  body,  and 
by  that  means  occasion  so  large  a  vacancy,  that  the 
grand  cloud  cannot  strike  in  that  place  ? ' 

§19.  Histoi^y  of  the  Leyden  Jar.    The  Ley  den  Battery. 

The  next  discovery  which  we  have  to  master  throws 

all  former  ones  into  the  shade.  It  was  first  announced  in 


Uiatory  of  the  Leyden  Jar.  65 

a  letter  addressed  on  the  4th  of  November,  1745,  to  Dr. 
Lieberkiihn,  of  Berlin,  by  Kleist,  a  clergyman  of 
Cammin,  in  Pomerania.  By  means  of  a  cork,  c,  fig.  34, 

FIG.  34. 


he  fixed  a  nail,  N,  in  a  phial,  G,  into  which  he  had  poured 
a  little  mercury,  spirits,  or  water,  w.  On  electrifying  the 
nail  he  was  able  to  pass  from  one  room  into  another  with 
the  phial  in  his  hand  and  to  ignite  spirits  of  wine  with  it. 
'If,'  said  he, '  while  it  is  electrifying  I  put  my  finger,  or  a 
piece  of  gold  which  I  hold  in  my  hand,  to  the  nail,  I 
receive  a  shock  which  stuns  my  arms  and  shoulders.' 

In  the  following  year  Cunseus  of  Leyden  made  sub- 
stantially the  same  discovery.  It  caused  great  wonder 
and  dread,  which  arose  chiefly  from  the  excited  imagi- 
nation. Musschenbroek  felt  the  shock,  and  declared 
in  a  letter  to  a  friend  that  he  would  iiot  take  a  second 
one  for  the  crown  of  France.  Bleeding  at  the  nose, 
ardent  fever,  a  heaviness  of  head  which  endured  for 
days,  were  all  ascribed  to  the  shock.  Boze  wished  that 
he  might  die  of  it,  so  that  he  might  enjoy  the  honour 
of  having  his  death  chronicled  in  the  Paris  '  Academy 


66  Lessons  in  Electricity. 

of  Sciences.'  Kleist  missed  the  explanation  of  the 
phenomenon  ;  while  the  Leyden  philosophers  correctly 
stated  the  conditions  necessary  to  the  success  of  the 
experiment.  Hence  the  phial  received  the  name  of  the 
Leyden  phial,  or  Leyden  jar. 

The  discovery  of  Kleist  and  Cunaeus  excited  the 
most  profound  interest,  and  the  subject  was  explored  in 
all  directions.  Wilson  in  1746  filled  a  phial  partially 
with  water,  and  plunged  it  into  water,  so  as  to  bring 
the  water  surfaces,  within  and  without  the  phial,  to  the 
same  level.  On  charging  such  a  phial  the  strength 
of  the  shock  was  found  greater  than  had  been  observed 
before. 

Two  years  subsequently  Dr.  Watson  and  Dr.  Bevis 
noticed  how  the  charge  grew  stronger  as  the  area  of  the 
conductor  in  contact  with  the  outer  surface  of  the  phial 
increased.  They  substituted  shot  for  water  inside 
the  jar,  and  obtained  substantially  the  same  effect. 
Dr.  Bevis  then  coated  a  plate  of  glass  on  both  sides 
with  silver  foil,  to  within  about  an  inch  of  the  edge,  and 
obtained  from  it  discharges  as  strong  as  those  obtained 
from  a  phial  containing  half  a  pint  of  water.  Finally 
Dr.  Watson  coated  his  phial  inside  and  out  with  silver 
foil.  By  these  steps  the  Leyden  jar  reached  the  form 
which  it  possesses  to-day. 

It  is  easy  to  repeat  the  experiment  of  Dr.  Bevis. 
Procure  a  glass  plate  nine  inches  square  ;  cover  it  on 
both  sides,  as  he  did,  with  tin-foil  seven  inches  square, 
leaving  the  rim  uncovered.  Connect  one  side  with  the 
earth  and  the  other  with  the  machine.  Charge  and  dis- 
charge :  you  obtain  a  brilliant  spark. 

In  our  experiment  with  the  Golden  Fish  (fig.  33),  we 
employed  a  common  form  of  the  Leyden  jar,  only  with 


History  of  the  Ley  den  Jar.  67 

the  difference  that  to  get  to  a  sufficient  distance  from 
the  glass,  so  as  to  avoid  the  attraction  of  the  fish  by 
the  jar  itself,  the  knob  was  placed  higher  than  usual. 
But  with  a  good  flint-glass  tumbler,  a  piece  of  tin-foil, 
and  a  bit  of  stout  wire,  you  can  make  a  jar  for  yourself. 
Bad  glass,  remember,  is  not  rare.1  In  fig.  35  you  have 

FIG.  35. 


such  a  jar.  T  is  the  outer,  Tf  the  inner  coating, 
reaching  to  within  an  inch  of  the  edge  of  the  tumbler 
G.  w  is  the  wire  fastened  below  by  wax,  and  sur- 
mounted by  a  knob,  which  may  be  of  metal,  or  of 
wax  or  wood,  coated  with  tin-foil.  In  charging  the 
jar  you  connect  the  outer  coating  with  the  earth — say 
with  a  gas-pipe  or  a  water-pipe — and  present  the  knob 
to  the  conductor  of  your  machine.  A  few  turns  will 
charge  the  jar.  It  is  discharged  by  laying  one  knob  of 
a  '  discharger '  against  the  outer  coating,  and  causing 
the  other  knob  to  approach  the  knob  of  the  jar.  Before 

1  In  preparing  these  Lessons  we  have  made  several  jars  which  re- 
fused to  be  charged,  through  the  badness  of  their  glass,  and  which 
ihowed  their  imperfect  insulation  by  discharging  our  electroscope. 


68 


Lessons  in  Electricity. 


FIG.  36. 


contact,  the  electricity  flies  from  knob  to  knob  in  the 

form  of  a  spark. 

A  '  discharger '  suited  to  our  means  and  purposes 
is  shown  in  fig.  36.  H  is  a  stick 
of  sealing-wax,  or,  better  still,  of 
ebonite :  w  w  a  stout  wire  bent  as 
in  the  figure,  and  ending  in  the 
knobs  B  B'.  These  may  be  of  wax 
coated  with  tin-foil.  Any  other 
light  conducting  knobs  would  of 
course  answer.  The  insulating 
handle  H  protects  you  effectually 
from  the  shock. 

You  must  render  yourself  expert 
in  the  use  of  the  discharger.     The 
mode  of  using  it  is  shown  in  fig.  37. 
FIG.  37. 


By  augmenting  the  size  of  a  Leyden  jar  we  render 


Explanation  of  the  Leyden  Jar. 


69 


it  capable  of  accepting  a  larger  charge  of  electricity. 
But  there  is  a  limit  to  the  size  of  a  jar.  When,  there- 
fore, larger  charges  are  required  than  a  single  jar  can 
furnish,  we  make  use  of  a  number  of  jars.  In  fig.  38 


FIG.  38. 


nine  of  them  are  shown.  All  their  interior  coatings 
are  united  together  by  brass  rods,  while  all  the  outer 
coatings  rest  upon  a  metal  surface  in  free  communica- 
tion with  the  earth. 

This  combination  of  Leyden  jars  constitutes  the 
Leyden  Battery,  the  effect  of  which  is  equal  to  that  of 
a  single  jar  of  nine  times  the  size  of  one  of  the  jars. 


§  20.  Explanation  of  the  Leyden  Jar. 

The  principles  of  electrical  induction  with  which 
you  are  now  so  familiar  will  enable  you  to  thoroughly 


70  Lessons  in  Electricity. 

analyse  and  understand  the  action  of  the  Leyden  jar. 
In  charging  the  jar  the  outer  coating  is  connected 
with  the  earth,  and  the  inner  coating  with  the  electrical 
machine.  Let  the  machine,  as  usual,  be  of  glass  yielding 
positive  electricity.  When  it  is  worked  the  electricity 
poured  into  the  jar  acts  inductively  across  the  glass 
upon  the  outer  coating ;  attracting  its  negative  and 
repelling  its  positive  to  the  earth.  Two  mutually  at- 
tractive electric  layers  are  thus  in  presence  of  each 
other,  being  separated  merely  by  the  glass.  When  the 
machine  is  in  good  order  and  the  glass  of  the  jar  is  thin, 
the  attraction  may  be  rendered  strong  enough  to  per- 
forate the  jar.  By  means  of  the  discharger  the  oppo- 
site electricities  are  enabled  to  unite  in  the  form  of  a 
spark. 

Franklin  saw  and  announced  with  clearness  the 
escape  of  the  electricity  from  the  outer  coating  of  the 
jar.  His  statement  is  that  whatever  be  the  quantity  of 
the  'electric  fire'  thrown  into  the  jar,  an  equal  quantity 
was  dislodged  from  the  outside.  We  have  now  to  prove 
by  actual  experiment  that  this  explanation  is  correct. 

Place  your  Leyden  jar  upon  a  table,  and  connect 
the  outer  coating  with  your  electroscope.  There  is  no 
divergence  of  the  leaves  when  electricity  is  poured  into 
the  jar. 

But  here  the  outer  coating  is  connected  through  the 
table  with  the  earth.  Let  us  cut  off  this  communica- 
tion by  an  insulator.  Place  the  jar  upon  a  board  sup- 
ported by  warm  tumblers,  or  upon  a  piece  of  vulcanised 
india-rubber  cloth,  and  again  connect  the  outer  coating 
with  the  electroscope.  The  moment  electricity  is  com- 
municated to  the  knob  of  the  jar  the  leaves  of  Dutch 
metal  diverge.  Detach  the  wire  by  your  discharger 


Explanation  of  the  Leyden  Jar. 


71 


and  test  the  quality  of  the  electricity — it  is  positive, 
us  theory  declares  it  must  be. 

Consider  now  the  experiment  of  Kleist  and  Cungeus 
(fig.  34).  You  will,  I  doubt  not,  penetrate  its  meaning. 
You  will  see  that  in  their  case  the  hand  formed  the 
outer  coating  of  the  jar.  When  electricity  was  com- 
municated through  the  nail  to  the  water  within,  that 
electricity  acted  across  the  glass  inductively  upon  the 
hand,  attracting  the  one  fluid  and  repelling  the  other 
to  the  earth. 

Again  I  say,  prove  ail  things ;  and  what  is  here 
affirmed  may  be  proved  by  the  following  beautiful  and 

FIG.  39. 


conclusive  experiment : — Stand  on  your  board, I r  ,hg.  39, 


72  Lesson?  in  Electricity. 

insulated  by  its  four  tumblers ;  or  upon  a  sheet  of 
gutta-percha,  or  vulcanised  india-rubber.  Seize  the  old 
Leyden  phial,  J,  with  your  left  hand,  and  present  the 
knuckle  of  your  right  hand  to  your  balanced  lath,  I/  L. 
When  electricity  is  communicated  to  the  nail,  the  lath 
is  immediately  attracted  by  the  knuckle.  Or  touch  your 
electroscope  with  your  right  hand :  when  the  phial  is 
charged  theleaves  immediately  diverge,  by  the  electricity 
driven  from  your  left  hand  to  the  electroscope. 

Here  the  nail  may  be  electrified  either  by  connect- 
ing it  with  the  prime  conductor  of  the  machine,  or  by 
rubbing  it  with  an  excited  glass  rod.  Indeed  I  should 
prefer  your  resorting  to  the  simplest  and  cheapest  means 
in  making  these  experiments. 


§  21.  Franklin's  Cascade.  Battery. 

As  a  thoughtful  and  reflective  boy  or  girl  you 
cannot,  I  think,  help  wondering  at  the  power  which  your 
thorough  mastery  of  the  principles  of  induction  gives 
you  over  these  wonderful  and  complicated  phenomena. 
By  those  principles  the  various  facts  of  our  science  are 
bound  together  into  an  organic  whole.  But  we  have 
not  yet  exhausted  the  fruitfulness  of  this  principle. 

Consider  the  following  problem.  Usually  we  allow 
the  electricity  of  the  outer  coating  to  escape  to  the  earth. 
Suppose  we  try  to  utilise  it.  Place,  then,  your  jar  A  B, 
fig.  40,  upon  vulcanised  india-rubber,  and  connect  by  a 
wire  B  c  its  outer  coating  with  the  knob  or  inner  coating 
of  a  second  jar  c  D.  What  will  occur  when  the  first  jar  is 
charged  ?  Why,  the  second  one  will  be  charged  also  by 
the  electricity  which  has  escaped  from  the  outer  coating 
of  the  first.  And  suppose  you  connect  the  outer  coat- 


Franklin's  Cascade  Battery.  73 

ing  of  the  second  insulated  jar  with  the  inner  coating  of 
a  third,  E  F  ;  what  occurs  ?  The  third  jar  will  obviously 
be  charged  with  the  electricity  repelled  from  the  outer 
coating  of  the  second.  Of  course  we  need  not  stop  here. 
We  may  have  a  long  series  of  insulated  jars,  the  outer 
coating  of  each  being  connected  with  the  inner  coating 
of  the  next  succeeding  one.  Connect  the  outer  coat- 
ing of  the  last  jar  I  K  by  a  wire  e  with  the  earth,  and 
charge  the  first  jar.  You  charge  thereby  the  entire 
series  of  jars.  In  this  simple  way  you  master  practi- 
Fio.  40. 


cally,  and   grasp  the  theory  of  Franklin's   celebrated 
'  cascade  battery.' 

You  must  see  that  before  making  this  important 
experiment  you  could  really  have  predicted  what  would 
occur.  This  power  of  prevision  is  one  of  the  most 
striking  characteristics  of  science. 


§  22.  Novel  Ley  den  Jars  of  the  Simplest 

Possessed    of   its    principles,    we    can    reduce    the 

Leyden  jar  to    far  simpler   forma    than   any  hitherto 


74  Le&sons  in  Electricity. 

dealt  with.  Spread  a  sheet  of  tin-foil  smoothly  upon  a 
table,  and  lay  upon  the  foil  a  pane  of  glass.  Remember 
that  the  glass,  as  usual,  must  be  dry.  Stick  on  to  the 
glass  by  sealing-wax  two  loops  of  narrow  silk  ribbon, 
by  which  the  pane  may  be  lifted ;  and  then  lay 
smoothly  upon  the  glass  a  second  sheet  of  tin-foil, 
less  than  the  pane  in  size,  leaving  a  rim  of  uncovered 
glass  all  round.  Carry  a  fine  wire  from  the  upper 
sheet  of  tin-foil  to  your  electroscope.  A  little  weight 
will  keep  the  end  of  the  wire  attached  to  the  tin-foil. 

Rub  this  weight  with  your  excited  glass  tube,  two 
or  three  times  if  necessary,  until  you  see  a  slight  diver- 
gence of  the  Dutch  metal  leaves.  Or  connecting  the 
weight  with  the  conductor  of  your  machine,  turn  very 
carefully  until  the  slight  divergence  is  observed.  What 
is  the  condition  of  things  here?  You  have  poured, 
say  positive  electricity  on  to  the  upper  sheet  of  metal. 
It  acts  inductively  across  the  glass  upon  the  under 
sheet,  the  positive  fluid  of  which  escapes  to  the 
earth,  leaving  the  negative  behind.  You  see  before  your 
mind's  eye  two  layers  holding  each  other  in  bondage. 
Now  take  hold  of  your  loops  and  lift  the  glass  plate,  so 
as  to  separate  the  upper  tin- foil  from  the  lower.  What 
would  you  expect  to  occur  ?  Freed  from  the  grasp  of 
the  lower  layer,  the  electricity  of  the  upper  one  will 
diffuse  itself  over  the  electroscope  so  promptly  and 
powerfully,  that  if  you  are  not  careful  you  will  destroy 
the  instrument  by  the  mutual  repulsion  of  its  leaves. 

Practise  this  experiment,  which  is  a  very  old  one 
of  mine,  by  lowering  and  lifting  the  glass  plate,  and 
observing  the  corresponding  rhythmic  action  of  the 
leaves  of  the  electroscope. 

Common  tin-plate  may  be  used  in  this  experiment 


Novel  Ley  den  Jars.  75 

instead  of  tin-foil,  and  a  sheet  of  vulcanised  india-rubber 
instead  of  the  pane  of  glass.  Or  simpler  still,  for  the 
tin-foil  a  sheet  of  common  unwarmed  foolscap  may  be 
employed.  Satisfy  yourself  of  this.  Spread  a  sheet  of 
foolscap  on  a  table  ;  lay  the  plate  of  glass  upon  it,  and 
spread  a  leaf  of  foolscap,  less  than  the  glass  in  size,  on  the 
plate  of  glass.  Connect  the  leaf  with  the  electroscope, 
and  charge  it,  exactly  as  you  charged  the  tin-foil.  On 
lifting  the  glass  with  its  leaf  of  foolscap,  the  leaves  of 
the  electroscope  instantly  fly  apart ;  on  lowering  the 
glass  they  again  fall  together.  Abandon  the  under 
sheet  altogether,  and  make  the  table  the  outer  coating ; 
if  it  be  not  of  very  dry  wood,  or  covered  by  an  insu- 
lating varnish,  you  will  obtain  with  it  the  results 
obtained  with  the  tin-foil,  tin,  and  foolscap.  Thus  by 
the  simplest  means  we  illustrate  great  principles. 

The  withdrawal  of  the  electricity  from  the  electro- 
scope, by  lowering  the  plate  of  glass,  so  as  to  bring  the 
electricity  of  the  upper  coating  within  the  grasp  of  the 
lower  one,  is  sometimes  called  '  condensation.'  The  elec- 
tricity on  one  plate  or  sheet  was  figured  as  squeezed 
together,  or  condensed,  by  the  attraction  of  the  other. 
A  special  instrument  called  a  condenser  is  constructed 
by  instrument  makers  to  illustrate  the  action  here  ex- 
plained. 

You  may  readily  make  a  condenser  for  yourself. 
Take  two  circles,  p  p7,  fig.  41,  of  tin  or  of  sheet  zinc, 
and  support  the  one,  p7,  by  a  stick  of  sealing-wax  or 
glass,  G  ;  the  other,  p,  by  a  metal  stem,  connected  with 
the  earth.  The  insulated  plate,  p',  is  called  the  col- 
lecting plate ;  the  uninsulated  one,  p,  the  condensing 
plate.  Connect  the  collecting  plate  with  your  electro- 


76 


Lessons  in  Electriciiy. 


scope  by  the  wire  w9  and  briDg  the  condensing  plate 
near  it,  leaving,  however,  a  thin  space  of  air  between 


FIG.  41. 


them.  Charge  the  collector,  p',  or  the  wire,  w,  with  your 
glass  rod,  until  the  leaves  of  the  electroscope  begin  to 
diverge.  Withdraw  the  condensing  plate,  the  leaves 
fly  asunder;  bring  the  condensing  plate  near,  the  leaves 

again  collapse. 

FIG.  42. 


Or  vary  your   construction,   and  make  your  con- 


Novel  Leyden  Jars.  77 

denser  tLus.  Employing  the  table,  or  a  sheet  of  foolscap 
if  the  table  be  an  insulator,  as  one  plate  of  the  con- 
denser, spread  upon  it  the  sheet  of  india-rubber,  p,  fig. 
42,  and  lay  upon  the  rubber  the  sheet  of  block-tin  A  B. 
Connect  the  tin  by  the  wire,  w,  with  the  electroscope, 
T.  Impart  electricity  to  the  little  weight,  A,  till  the 
leaves,  L,  begin  to  diverge ;  then  lift  the  tin-plate  by 
its  two  silk  loops  ;  the  leaves  at  once  fly  asunder. 

Finally  show  your  complete  knowledge  of  the  Leyden 
jar,  and  your  freedom  from  the  routine  of  the  instru- 
ment makers,  by  making  a  'jar'  in  the  following  novel 
way.  Stand  upon  a  board  supported  by  warm  tumblers. 
Hold  in  your  right  hand  a  sheet  of  vulcanised  india- 
rubber,  and  clasp,  with  it  between  you,  the  left  hand 
of  a  friend  in  connection  with  the  earth.  Place  your 
left  hand  on  the  conductor  of  the  machine,  and  let  it 
be  worked.  You  and  your  friend  soon  feel  a  crackling 
and  a  tickling  of  the  hands,  due  to  the  heightening  at- 
traction of  the  opposite  electricities  across  the  india- 
rubber.  The  '  hand-jar '  is  then  charged.  To  discharge 
it  you  have  only  to  bring  your  other  hands  together : 
the  shock  of  the  Leyden  jar  is  then  felt  and  its  spark 
seen  and  heard. 

By  the  discharge  of  the  hand-jar  you  can  fire  gun- 
powder. But  this  will  be  referred  to  more  particularly 
further  on.  (See  §  25.) 


§  23.  Seat  of  Charge  in  the  Leyden  Jar* 

Franklin  sought  to  determine  how  the  charge  was 
hidden  in  the  Leyden  jar.  He  charged  with  electricity 
a  bottle  half-filled  with  water  and  coated  on  the  outside 
with  tin-foil :  dipping  the  finger  of  one  hand  into  the 


78  Lessons  in  Electricity. 

water,  and  touching  the  outside  coating  with  the  ether, 
he  received  a  shock.  He  was  thus  led  to  inquire,  is  the 
electricity  in  the  water?  He  poured  the  water  into  a 
second  bottle,  examined  it,  and  found  that  it  had  car- 
ried no  electricity  along  with  it. 

His  conclusion  was  '  that  the  electric  fire  must 
either  have  been  lost  in  the  decanting,  or  must  have 
remained  in  the  bottle.  The  latter  he  found  to  be 
true ;  for,  filling  the  charged  bottle  with  fresh  water,  he 
Dbtained  the  shock,  and  was  therefore  satisfied  that  the 
power  of  giving  it  resided  in  the  glass  itself.' l 

(An  account  of  Franklin's  discoveries  was  given  by 
him  in  a  series  of  letters  addressed  to  Peter  Collinson, 
Esq.,  F.K.S.,  from  1747  to  1754.) 

So  much  for  history ;  but  you  are  to  verify  the  his- 
tory by  repeating  Franklin's  experiments.  Place  water 
in  a  wide  glass  vessel ;  place  a  second  glass  vessel  within 
the  first,  and  fill  it  to  the  same  height  with  water. 
Connect  the  outer  water  by  a  wire  with  the  earth,  and 
the  inner  water  by  a  wire  with  the  electric  machine. 
One  or  two  turns  furnish  a  sufficient  charge.  Re- 
moving the  inner  wire,  and  dipping  one  finger  into 
the  outside  and  the  other  into  the  inside  water,  a 
smart  shock  is  felt.  This  was  Franklin's  first  ex- 
periment. 

Pass  on  to  the  second.  Coat  a  glass  jar  with  tin-foil 
(not  too  high) ;  fill  it  to  the  same  height  with  water,  and 
place  it  on  india-rubber  cloth.  Charge  it  by  con- 
necting the  outside  ccating  with  the  earth,  and  the 
water  inside  (by  means  of  a  stem  cemented  to  the  bottom 
of  the  jar  and  ending  above  in  a  knob)  with  an  electric 

1  Priestley's  'History  of  Electricity,  3rd  edition,  p.  149. 


Seat  of  Charge  in  the  Leyden  Jar. 


'9 


machine.     You  obtain  a  bright  spark  on  discharging. 
This  proves  your  apparatus  to  be  in  good  order. 

Ee-charge.  Take  hold  of  the  charged  jar  with  the 
india-rubber,  and  pour  the  water  into  a  second  similar 
jar.  No  sensible  charge  is  imparted  to  the  latter.  Pour 
fresh  unelectrified  water  into  the  first  jar,  and  discharge 
it.  The  retention  of  the  charge  is  shown  by  a  brilliant 
spark.  Be  careful  in  these  experiments,  or  you  will 
fail  as  I  did  at  first.  The  edge  of  the  jar  out  of  which 
the  water  is  poured  has  to  be  surrounded  by  a  band  of 
bibulous  paper  to  catch  the  final  drop,  which,  trickling 
down,  would  discharge  the  jar. 

Experiments  like  those  of  Franklin  are  now  made  by 
rendering  the  coatings  of  the  Leyden  jar  movable.  Such 
a  jar  being  charged,  the  interior  coating         FIG  43, 
may  be  lifted  out  and  proved  unelectric. 
The   glass  may  then   be   removed  from 
the  outer  coating  and  the  latter  proved  un- 
electric.    Eestoring  the  jar  and  coatings, 
on   connecting  the   two  latter,  the   dis- 
charge passes  in  a  brilliant  spark. 

Make  a  jar  with  movable  coatings 
thus  : — Eoll  cartridge  paper  round  a  good 
flint-glass  tumbler,  G,  fig.  43,  to  within 
about  an  inch  of  the  top.  Paste  down  the 
lower  edge  of  the  paper,  and  put  a  paper 
bottom  to  it  corresponding  to  the  bottom 
of  the  glass.  Coat  the  paper,  T,  inside 
and  out  with  tin-foil.  Make  a  similar 
coating,  T',  for  the  inside  of  the  tum- 
bler, attaching  to  it  an  upright  wire,  w, 
ending  in  a  hook.  You  have  then  to  all  intents  and 
purposes  a  Leyden  jar. 


80  Lessons  in  Electricity. 

Put  the  pieces  together  and  charge  the  jar.  By 
means  of  a  rod  of  glass,  sealing-wax,  or  gutta-percha, 
lift  out  the  interior  coating.  It  will  carry  a  little 
electricity  away  with  it.  Place  it  upon  a  table  and 
discharge  it  wholly.  Then  by  the  hand  lift  the  glass 
out  of  the  outer  coating.  Neither  of  the  coatings  now 
shows  the  slightest  symptom  of  electricity.  Restore  the 
tumbler  to  its  outer  coating,  and,  by  means  of  the  hook 
and  insulating  rod,  restore  the  inner  coating  to  its  place. 
Discharge  the  jar  :  you  obtain  a  brilliant  spark.  The 
electricity  which  produces  this  spark  must  have  been 
resident  in  and  on  the  glass. 

Here,  as  in  all  other  cases,  you  can  charge  your  jar 
with  a  rubbed  glass  tube,  though  a  machine  in  good 
working  order  will  do  it  more  rapidly.  With  f  Cot- 
trell's  rubber,'  described  in  the  next  section,  you  may 
greatly  exalt  the  performance  of  your  glass  tube. 

§  24.  Ignition  by  the  Electric  Spark.     CottrelL's 
Rubber.     The  Tube-machine. 

Various  attempts  had  been  vainly  made  by  Nollet 
and  others  to  ignite  inflammable  substances  by  the 
electric  spark.  This  was  first  effected  by  Ludolf,  at 
the  opening  of  the  Academy  of  Sciences  by  Frederick 
the  Great  at  Berlin,  on  the  23rd  of  January,  1744. 
With  a  spark  from  the  sword  of  one  of  the  court 
cavaliers  present  on  the  occasion,  Ludolf  ignited 
sulphuric  ether. 

Dr.  Watson  also  made  numerous  experiments  on 
the  ignition  of  bodies  by  the  electric  spark.  He  fired 
gunpowder  and  discharged  guns.  Causing,  moreover,  a 
spoon  containing  ether  to  be  held  by  an  electrified  person, 


Ignition  by  the  Electric  Spark.  81 

he  ignited  the  ether  by  the  finger  of  an  unelectrified 
person.  He  also  noticed  that  the  spark  varied  in  colour 
when  the  substances  between  which  it  passed  varied. 

These,  and  numerous  other  experiments  may  be 
made  with  a  far  simpler  '  machine '  than  any  hitherto 
described.  It  was  devised  for  your  benefit  by  Mr. 
Cottrell.  In  the  electric  machine,  as  we  have  learned, 
the  prime  conductor  is  flooded  with  positive  electricity 
through  the  discharge  of  the  negative  from  the  points 
against  the  excited  glass.  Your  glass  tube  and  rubber 
may  be  similarly  turned  to  account.  A  strip  of  sheet- 
brass  or  copper,  p,  fig.  44,  is  sewn  on  to  the  edge  of 

Fio.  44 


the  silk  pad,  R,  employed  as  a  rubber.  Through  aper- 
tures in  the  strip  about  twenty  pin-points  are  intro- 
duced, and  soldered  to  the  metal.  When  the  tube  is 
clasped  by  the  rubber,  the  metal  strip  and  points  quite 
encircle  the  tube. 

When  a  fine  wire,  w,  connects  the  strip  of  metal  with 
the  knob  of  a  Ley  den  jar,  by  every  downward  stroke 
of  the  rubber  the  glass  tube  is  powerfully  excited,  and 


82 


Lessons  in  Electricity. 


hotly  following  the  exciting  rubber  is  the  circle  of  points. 
From  these,  against  the  rod,  negative  electricity  is  dis- 
charged, the  free  positive  electricity  escaping  along  the 
wire  to  the  jar,  which  is  thus  rapidly  charged. 

The  ignition  of  gas  is  readily  effected  by  Cottrell's 
rubber.  Connecting  the  strip  of  metal,  R,  fig.  45,  with  an 
insulated  metallic  knob,  B,  placed  within  a  quarter  or  an 
eighth  of  an  inch  of  an  uninsulated  argand  burner 
connected  with  the  earth,  at  every  downward  stroke  of 
the  rubber  a  stream  of  sparks  passes  between  the  knob 

FIG.  45. 


and  burner.  If  gas  be  turned  on,  it  is  immediately 
ignited  by  the  stream  of  sparks.  Blowing  out  the  flame 
and  repeating  the  experiment,  every  stroke  of  the 
rubber  infallibly  ignites  the  gas. 

Sulphuric  ether,  in  a  spoon  which  has  been  previously 
warmed,  is  thus  ignited :  but  the  ether  soon  cools  by 
evaporation  ;  its  vapour  is  diminished  by  the  cold,  and 
it  is  then  less  easy  to  ignite.  Bisulphide  of  carbon 
may  be  substituted  for  the  ether,  with  the  certainty 


Cotireil's  Rubber. 


83 


that  every  stroke  of  the  rubber  will  set  it  ablaze.1 
The  spark  thus  obtained  also  fires  a  mixture  of 
oxygen  and  hydrogen.  The  two  gases  unite  with  ex- 
plosion to  form  water,  when  an  electric  spark  is  passed 
through  them. 

Mr.  Cottrell  has  also  mounted  his  glass  tube  so  as  to 
render  friction  in  both  directions  available.  The  tube- 
machine  is  represented  in  fig.  46.  A  B  is  the  glass 

Fio.  46. 


tube,  clasped  by  the  rubber,  R.  p  p7  are  two  strips  of 
metal  furnished  with  rows  of  points.  From  p  V  wires 
proceed  to  the  knob  c,  which  is  insulated  by  the 
horizontal  stem,  G.  This  insulating  stem  may  be 
abolished  with  advantage,  the  wires  from  p  and  P' 
being  rendered  strong  enough  to  support  the  ball  c. 

i  am  indebted  to  Dr.  Debus  for  the  suggostion  of  the  bisulphide 
as  a  substitute  for  the  ether. 


84  Lessons  in  Electricity. 

At  c  sparks  may  be  taken,  a  Leyden  jar  charged,  the 
electric  mill  turned,  while  wires  carried  from  it  may  be 
employed  in  experiments  on  ignition.  I  however 
strongly  recommend  to  your  attention  the  more  simple 
rubber  shown  in  tig.  44. 

'  Seldom,'  says  Riess,  '  has  an  experiment  done  so 
much  to  develope  the  science  to  which  it  belongs  as 
(his  of  the  ignition  of  bodies  by  the  electric  spark.' 
It  aroused  universal  interest ;  and  was  repeated  in  all 
Royal  houses.  Money  was  ready  for  the  further  pro- 
secution of  electrical  research.  The  experiment  after- 
wards spread  among  the  people.  Riess  considers  it 
probable  that  the  general  interest  thus  excited  led  to 
the  discovery  of  the  Leyden  jar,  which  was  made  soon 
afterwards. 

Klingenstierna  astonished  King  Frederick  of  Sweden 
by  igniting  a  spoon  of  alcohol  with  a  piece  of  ice. 
With  Cottrell's  rubber  and  bisulphide  of  carbon  this 
striking  experiment  is  easily  made,  and  you  ought  to 
render  your  knowledge  complete  by  repeating  it.  At 
every  stroke  of  the  rubber  the  spark  from  the  end  of  a 
pointed  rod  of  ice  infallibly  sets  the  bisulphide  on  fire. 

Cadogan  Morgan,  in  1785,  sought  to  produce  the 
electric  spark  in  the  interior  of  solid  bodies.  He 
inserted  two  wires  into  wood,  and  caused  the  spark  to 
pass  between  them :  the  wood  was  illuminated  with 
blood-red  light,  or  with  yellow  light,  according  as  the 
depth  at  which  the  spark  was  produced  was  greater  or 
less.  The  spark  of  the  Leyden  jar  produced  within  an 
ivory  ball,  an  orange,  an  apple,  or  under  the  thumb, 
illuminates  these  bodies  throughout.  A  lemon  is  espe- 
cially suited  to  this  experiment;  flashing  forth  at  every 
Bpark  as  a  spheroid  of  brilliant  golden  light.  The 


Duration  of  the  Electric  Spark. 


85 


manner  in  which  the  lemon  is  mounted  on  the  brass 
stem  B  is  shown  in  fig.  47.  The  spark  occurs  at  8, 
in  the  interval  between  the  stems  A  and  B.  A  row  of 


eggs  in  a  glass  cylinder  is  also  brilliantly  illuminated 
at  the  passage  of  every  spark  from  a  Leyden  jar. 

§  25.  Duration  of  the  Electric  Spark. 

The  duration  of  the  electric  spark  is  very  brief:  in 
a  special  case,  Sir  Charles  Wheatstone  found  it  to  be 
? 4  o  o  otb  °f  a  second.  This,  however,  was  the  maximum 
duration.  In  other  cases  it  was  less  than  the  millionth 
of  a  second. 

When  a  body  is  illuminated  for  an  instant,  the 
image  of  the  body  remains  upon  the  retina  of  the  eye 
for  about  one-fifth  of  a  second.  If,  then,  a  body  in  swift 
motion  be  illuminated  by  an  instantaneous  flash,  it 
will  be  seen  to  stand  motionless  for  one-fifth  of  a 
second  at  the  point  where  the  flash  falls  upon  it.  A 
rifle  bullet  passing  through  the  air,  and  illuminated  by 


86  Lessons  in  Electricity. 

an  electric  flash,  would  be  seen  thus  motionless ;  a 
circle  like  D  D',  fig.  48,  divided  into  black  and  white 
sectors,  and  rotating  so  quickly  as  to  cause  the  sectors 

FIG.  48. 


to  blend  to  a  uniform  grey,  appears,  when  illuminated 
by  the  spark  of  a  Ley  den  jar,  perfectly  motionless,  with 
all  its  sectors  revealed.  A  falling  jet  of  water,  which 
appears  continuous,  is  resolved  by  the  electric  flash  into 
its  constituent  drops.  Lightning,  as  shown  by  Professor 
Dove,  is  similarly  rapid  in  its  discharge. 

For  a  long  time  it  was  found  almost  impossible  to 
ignite  gunpowder  by  the  electric  spark.  Its  duration  is 
so  brief  that  the  powder,  when  the  discharge  occurred  in 
its  midst,  was  simply  scattered  violently  about.  In  1 787 
Wolff  introduced  into  the  circuit  through  which  the 
discharge  passed  a  glass  tube  wetted  on  the  inside.  He 
thereby  rendered  the  ignition  certain.  This  was  owing  to 
the  retardation  of  the  spark  by  the  imperfect  conductor. 
Gun-cotton,  phosphorus,  and  amadou,  which  are  torn 
asunder  by  the  unretarded  spark,  are  ignited  when  the 
discharge  is  retarded  by  a  tube  of  water.  A  wetted 
string  is  the  usual  means  resorted  to  for  retardation 
when  gunpowder  is  to  be  discharged. 


Duration  of  the  Electric  Spark. 


8? 


The  instrument  usually  employed  for  the  ignition 
of  powder  is  the  universal  discharger.  We  make  our 
own  discharger  thus  : — I  and  i'  (fig.  49)  are  insulating 


Fio.  49. 


rods  of  glass  or  sealing-wax,  supporting  two  metal  arms, 
the  ends  of  which  can  be  brought  down  upon  the  little 
central  table  s.  One  of  the  metal  arms  of  the  discharger 
being  connected  by  a  wire  e  with  the  earth,  the  sepa- 
rated ends  of  the  two  arms  are  surrounded  with  powder 
at  s.  Sending  through,  it  the  unretarded  charge,  the 
powder  is  scattered  mechanically.  Introducing  the  wet 
string  w  into  the  circuit,  ignition  infallibly  occurs  when 
the  spark  passes. 

This  is  the  place  to  fulfil  our  promise  to  ignite  gun- 
powder by  the  (  hand-jar.'  Fig.  50  explains  the  arrange- 
ment. H  H'  are  the  hands  of  the  insulated  person.  F 


88 


Lessons  in  Electricity. 


the  hand  of  the  uninsulated  friend,  I  the  india-rubber 

between  both  hands.    The  lead  ball  B  is  suspended  by 

a  wet  string  s.     On  the  little  stand  r,  connected  with 

Fio.  50. 


n 

the  earth,  is  placed  the  powder.  The  charging  of  the 
hand-jar  is  described  in  §  22.  When  charged,  it  is 
only  necessary  to  bring  the  ball  B  down  upon  the  powder 
to  cause  it  to  explode. 


§  26.  Electric  Light  in  Vacuo. 

The  electric  light  in  vacuo  was  first  observed  by 
Picard  in  1675.  While  carrying  a  barometer  from  the 
Observatory  to  the  Porte  St.  Michel  in  Paris,  he  saw 
light  in  the  upper  portion  of  the  tube.  Sebastien  and 
Cassini  observed  it  afterwards  in  other  barometers.  John 
Bernouilli  devised  a  '  mercurial  phosphorus,'  by  shaking 
mercury  in  a  tube  which  had  been  exhausted  by  a^ 


Electric  Light  in  Vacuo. 


b'J 


FIG.  51. 


air-pump.  This  was  Landed  to  the  King  of  Prussia — 
Frederick  I. — who  awarded  for  it  a  medal  of  fortv 
ducats  value.  The  great  mathematician  wrote  a  poem 
in  honour  of  the  occasion. 

Bernouilli  failed  to  explain  the  effect.  The  ex- 
planation was  reserved  for  Haukshee,  who  in  1705  took 
up  the  subject  and  experimented  upon  it  before  the 
Koyal  Society.  On  the  plate  of  an  air-pump  he  placed 
two  bell-jars,  one  over  the  other.  The  outer  and  larger 
jar  was  open  at  the  top.  Into  the  opening  Hauksbee 
fixed,  air-tight,  a  funnel,  which  he  stopped  with  a  plug  of 
wood  and  filled  with  mercury.  He  exhausted  the  space 
between  the  two  jars,  withdrew 
the  wooden  plug  and  allowed 
the  mercury  to  stream  against 
the  outer  surface  of  the  inner  jar. 
He  thus  obtained  a  shower  of  fire. 
This  is  a  truly  beautiful  experi- 
ment when  witnessed  by  an  ob- 
server close  at  hand. 

A  copy  of  Hauksbee's  own 
figure  illustrating  this  experiment 
is  annexed,  fig.  51.  M  is  the  funnel 
containing  the  mercury,  p  the  plug 
of  wood,  s  the  outer  and  s'  the  in- 
ner bell-jar.  Instead  of  the  plug 
P,  an  india-rubber  tube,  held  by  a 
clip,  may  be  employed  with  ad- 
vantage to  connect  the  funnel 
with  the  exhausted  jar.  By 
gradually  relaxing  the  clip  the 

mercury  may  be  made  to  fall  at  a  rate  corresponding 
to  the  maximum  luminous  effect.  The  streams  of  light 


90  Lessons  in  Electricity. 

pioduced  are  very  beautiful,  but  they  are  more  con- 
tinuous than  they  are  shown  to  be  by  Hauksbee. 

In  1706  HauKsbee  referred  the  phenomenon  to  its 
true  cause,  namely,  the  friction  between  mercury  and 
glass  in  the  highly  rarefied  air.  John  Bernouilli  ridi- 
culed Hauksbee's  explanation.  But  trutli  outlives 
ridicule,  and  it  is  now  universally  admitted  that 
Hauksbee  was  right. 

Hauksbee  also  made  the  following  experiment, 
which,  as  shown  by  Eiess,  is  explained  by  reference 
to  the  principle  of  induction.  A  hollow  glass  globe 
was  mounted  so  as  to  be  capable  of  quick  rotation. 
It  was  exhausted,  and  while  it  rotated  the  hand 
was  placed  against  it  in  the  dark.  It  was  positively 
electrified  by  the  hand.  This  positive  electricity  acted 
inductively  on  the  glass  itself,  attracting  its  negative, 
but  discharging  its  positive  as  a  luminous  glow  through' 
the  rarefied  air  within.  Hauksbee  was  able  to  read  by 
the  light  thus  produced. 

By  such  experiments  it  was  shown  that  rarefied  air 
favoured  the  passage  of  electricity.  Dry  air  is  in  fact 
an  insulator,  which  must  be  broken  through  to  produce 
the  electric  spark.  Through  an  exhausted  glass  tube 
six  feet  long  a  discharge  freely  passes  which  would 
be  incompetent  to  leap  over  the  fiftieth  part  of  this 
interval  in  air.  But  whereas  the  spark  in  air  is  dense 
and  brilliant,  the  discharge  in  vacuo  fills  the  exhaust<  d 
tube  with  a  diffuse  light. 

(It  is  here  worthy  of  remark  that  at  a  very  early 
period  Grummert,  a  Pole,  proposed  the  empl6yment 
of  this  diffuse  electric  light  to  illuminate  coal  mines — 
a  notion  which  has  been  revived  in  our  day.  The  light 


Electric  Light  in   Vacua.  91 

.n  this  form  is  not  competent  to  ignite  the  explosive 
gases  which  produce  such  terrible  disasters  in  mines.) 

Priestley,  in  his  •  History  of  Electricity,'  thus  de- 
scribes the  light  in  vacuo.  '  Take  a  tall  receiver,  very 
dry,  and  in  the  top  of  it  insert  with  cement  a  wire  not 
very  acutely  pointed,  then  exhaust  the  receiver  and  pre- 
sent the  knob  of  the  wire  to  the  conductor,  and  every 
spark  will  pass  through  the  vacuum  in  a  broad  stream  of 
light,  visible  through  the  whole  length  of  the  receiver, 
be  it  ever  so  tall.  This  stream  often  divides  itself  into 
a  variety  of  beautiful  rivulets,  which  are  continually 
changing  their  course,  uniting  and  dividing  again  in 
the  most  pleasing  manner.  If  a  jar  be  discharged 
through  this  vacuum,  it  gives  the  appearance  of  a  very 
dense  body  of  fire,  darting  directly  through  the  centre 
of  the  vacuum  without  ever  touching  the  sides.' 

Cavendish  employed  a  double  barometer-tube,  bent 
into  the  form  of  a  horseshoe,  with  its  curved  portion 
empty,  to  show  the  passage  of  electricity  through  a 
vacuum.  It  is  really  not  the  vacuum  which  con- 
ducts the  electricity,  but  the  highly  attenuated  air  and 
vapour  which  fill  the  space  above  the  barometric 
columns.  When  the  mercury  employed  is  carefully 
purged  of  air  and  moisture  by  previous  boiling,  the  space 
above  the  mercury,  as  proved  by  Walsh,  De  Luc,  Morgan, 
and  Davy,  is  wholly  incapable  of  conducting  electricity. 
Similar  experiments  have  been  made  in  the  laboratory 
of  Mr.  Grassiot,  to  whom  we  are  indebted  for  so  many 
beautiful  electrical  experiments.  Professor  Dewar  has 
also  brought  his  experimental  skill  to  bear  with  success 
upon  this  subject. 

Electricity  therefore  does  not  pass  through  a  true 
:  it  requires  ponderable  matter  to  carry  it.  If 


92  Lessons  in  Electricity. 

a  gold-leaf  electroscope  be  kept  at  a  distance  from  ail 
conductors,  it  may  be  kept  charged  for  an  almost  in- 
definite period  in  a  good  air-pump  vacuum. 

The  matter  rendered  thus  luminous  by  the  electrical 
discharge  is  attracted  and  repelled  like  other  electrified 
matter.  '  A  finger,'  says  Priestley,  '  put  on  the  outside 
of  the  glass  will  draw  it  [the  luminous  stream]  wherever 
a  person  pleases.  If  the  vessel  be  grasped  with  both 
hands,  every  spark  is  felt  like  the  pulsation  of  a  great 
artery,  and  all  the  fire  makes  towards  the  hands.  This 
pulsation  is  felt  at  some  distance  from  the  receiver ;  and 
in  the  dark  a  light  is  seen  betwixt  the  hands  and  glass.' 

'  All  this,'  continues  the  historian  of  Electricity, 
'  while  the  pointed  wire  is  supposed  to  be  electrified 
positively  ;  if  it  be  electrified  negatively  the  appearance 
is  remarkably  different.  Instead  of  streams  of  fire, 
nothing  is  seen  but  one  uniform  luminous  appearance, 
like  a  white  cloud,  or  the  milky-way  on  a  clear  star- 
light night.  It  seldom  reaches  the  whole  length  of 
the  vessel,  but  is  generally  only  like  a  lucid  ball  at  the 
end  of  the  wire.' 

Of  the  two  appearances  here  described  the  former  is 
now  known  as  the  electric  brush,  and  the  latter  as  the 
electric  gloiv.  Both  can  be  produced  in  unconfined  air. 
The  glow  is  sometimes  seen  on  the  masts  of  ships,  and  it 
is  mentioned  by  the  ancients  as  appearing  on  the  points 
of  lances.  It  is  called  St.  Ermo's  or  St.  Elmo's  fire,  after 
the  sailor's  saint,  Erasmus,  who  suffered  martyrdom  at 
Gaeta  at  the  beginning  of  the  fourth  century. 

The  purple  colour  of  the  diffused  light  in  attenuated 
air  was  noticed  by  Hauksbee.  The  colour  depends 
upon  the  residue  of  attenuated  gas,  or  vapour,  through 
which  the  discharge  passes.  If  it  be  an  oxygen-residue 


Electric  Light  in  Vacua. 


93 


the  light  is  whitish,  if  it  be  a  hydrogen-residue  the  light 
is  red,  if  a  nit'.ogen-residue  the  light  is  purple,  exactly 
resembling  that  displayed  at  times  by  the  aurora  bore- 
alis — a  colour  doubtless  due  to  the  discharge  of  elec- 
tricity through  the  attenuated  nitrogen  of  the  air. 

Electric  light  in  vacuo  is  readily  produced  by  the 
fritstion  of  an  amalgamated  rubber  against  the  outside 

FIG.  52. 


Di  an  exhausted  tube.  The  light  also  is  produced  by 
the  friction  of  mercury  within  a  barometric  vacuum. 
The  discharges  through  tubes  many  feet  in  length  and 


94  Lessons  in  Electricity. 

exhausted  by  an  air-pump  are  very  fine.  The  double, 
barometer  tube  of  Cavendish  also  yields  a  truly  splendid 
bow  of  light,  when  a  strong  electric  discharge  is  sent 
through  it.  For  this  experiment  fig.  52  shows  the  best 
arrangement  p  is  the  prime  conductor  of  an  electrical 
machine,  I  an  insulated  metal  ball,  connected  by  a  wire 
with  the  mercury  trough  A.  The  trough  B  is  connected 
by  a  wire  with  the  earth,  c  and  (/  mark  the  height  of 
the  mercurial  columns.  When  the  machine  is  worked 
sparks  pass  from  p  to  I,  a  vivid  bow  of  light  at  each 
passage  stretching  from  c  to  c'.  By  causing  I  to  ap- 
proach p,  the  discharges  become  more  frequent,  but 
more  feeble ;  by  augmenting  the  distance  p  I,  the 
sparks  become  rarer,  but  more  strong.  When  very 
strong,  a  bow  of  dazzling  brilliancy  accompanies  every 
spark.1 

Small  tubes  for  these  experiments  are  best  obtained 
from  philosophical  instrument  makers. 

§  27.  Lichtenberg's  Figures. 

Lichtenberg  devised  a  means  of  revealing  the  con- 
dition of  an  electrified  surface  by  dusting  it  with  powder. 
Ked  lead,  in  passing  through  muslin,  is  positively  elec- 
trified ;  flower  of  sulphur  is  negatively  electrified. 
Whisking  a  fox's  brush  over  a  cake  of  resin,  and  draw- 
ing over  the  surface  the  knob  of  a  Leyden  jar,  positively 
charged,  the  resin  is  rendered  in  part  negative  and  in 
part  positive.  Dusting  the  mixed  powder  over  the  sur- 
face, the  sulphur  arranges  itself  over  the  positive  places, 
and  the  red  lead  over  the  negative  places,  a  very  beauti- 
ful pattern  being  the  result. 

'  It  is  well  to  hare  the  interval  PI  at  some  distance  from  the  bow, 
so  that  the  light  of  the  spark  shall  not  impair  the  effect  of  the  disehargs 
•ipon  the  eye. 


Surface  Compared  with  Mass.  95 

This  experiment  of  Lichtenberg's  constituted  the 
germ  of  Chladni's  important  acoustical  researches. 
'  Chladni's  figures'  were  the  direct  offspring  of  '  Lich- 
tenberg's  figures.' 


§  28.  Surface  Compared  with  Mass.     Distribution 
of  Electricity  in  Hollow  Conductors. 

Monnier  proved  that  the  charge  of  a  conductor  de- 
pended upon  its  surface,  and  not  upon  its  solid  contents. 
An  anvil  weighing  200  Ibs.  gave  a  smaller  spark  than 
a  speaking  trumpet  weighing  10  Ibs.  A  solid  ball  of 
lead  gave  a  spark  only  of  the  same  force  as  that  obtained 
from  a  piece  of  thin  lead  of  the  same  superficies,  bent 
into  the  form  of  a  hoop.  Finally  Monnier  obtained  a 
strong  spark  from  a  long  strip  of  sheet  lead,  but  a  very 
umall  one  when  it  was  rolled  into  a  lump. 

Le  Eoi  and  D'Arcy  showed  that  a  hollow  sphere 
accepted  the  same  charge  when  empty  as  when  filled 
with  mercury,  which  augmented  its  weight  60-fold. 
All  this  proves  the  influence  of  surface  as  distinguished 
from  mass. 

The  distribution  of  electric! tyMs  well  illustrated  Ly 
the  deportment  of  hollow  bodies.  Impart  by  your 
carrier  (fig.  15)  successive  measures  of  electricity  to  the 
interior  of  an  insulated  ice-pail,  or  a  pewter  pot.  On 
testing  the  interior  of  the  vessel  with  the  carrier  and 
an  electroscope  no  electricity  is  found  there  ;  but  it  is 
found  on  the  external  surface.  A  hat  suspended  by 
Bilk  strings  answers  as  well  as  the  ice-pail. 

This  experiment  with  the  hat  is  a  very  instructive 
one.  The  hat  may  be  charged  either  with  Cottrell's 
rubber  or  with  your  rubbed  glass  tube. 


96 


Lessons  in  Electricitn. 


Notice,  when  testing,  that  you  take  your  strongest 
charges  from  tbe  edges  and  not  from  the  round  or  H;it 
surface  of  the  hat.  The  strongest  charge  of  all  is  com- 
municated to  the  carrier  by  the  leaf  of  the  hat. 

The  successive  charges  may  be  communicated  to 
the  hat  by  a  metal  ball  suspended  by  silk.  The  charged 
ball,  on  touching  the  interior  surface,  becomes  com- 
pletely  unelectric. 


Franklin  placed  a  long  chain  in  a  silver  teapot 
which  he  electrified.  Connecting  his  teapot  with  a 
pith-ball  electroscope  he  produced  a  divergence.  Then 
lifting  the  chain  by  a  silk  string  he  found  that  over 


' 


Physiological  Effects  of  the  Electrical  Discharge.     97 

the  portion  outside  the  teapot  the  electricity  diffused 
itself,  this  withdrawal  of  the  electricity  from  the 
electroscope  being  announced  by  the  partial  collapse 
of  the  divergent  pith-balls. 

The  mode  of  repeating  this  experiment  is  shown  in 
Sg.  53,  where  T  is  the  teapot,  supported  on  a  good 
glass  tumbler  G,  and  connected  by  the  wire  w  with  the 
electroscope  E.  The  effect  is  small,  but  distinct. 

The  greatest  experiment  with  hollow  conductors  was 
made  by  Faraday,  who  placed  himself  in  a  cubical 
chamber  built  of  laths  and  covered  with  paper  and  wire 
gauze.  It  was  suspended  by  silk  ropes.  Within  this 
chamber  he  could  not  detect  the  slightest  sign  of  elec- 
tricity, however  delicate  his  electroscope,  and  however 
strongly  the  sides  of  the  chamber  might  be  electrified. 


§  29.  Physiological  Effects  of  the  Electric  Discharge. 

The  physiological  effect  of  the  electric  shock  has 
been  studied  in  various  ways.  Graham  caused  a  number 
of  persons  to  lay  hold  of  the  same  metal  plate,  which  was 
connected  with  the  outer  coating  of  a  charged  Leyden 
jar,  and  also  to  lay  hold  of  a  rod  by  which  the  jar  was  dis- 
charged. The  shock  divided  itself  equally  among  them. 

The  Abbe  Nollet  formed  a  line  of  one  hundred  and 
eighty  guardsmen,  and  sent  the  discharge  through  them 
all.  He  also  killed  sparrows  and  fishes  by  the  shock. 
The  analogy  of  these  effects  with  those  produced  by 
thunder  and  lightning  could  not  escape  attention,  nor 
fail  to  stimulate  enquiry. 

Indeed,  as  experimental  knowledge  increased,  men's 
thoughts  became  more  definite  and  exact  as  regards  the 
relation  of  electrical  effects  to  thunder  and  lightning. 


98  Lessons  in  Electricity. 

The  Abbe  Nollet  thus  quaintly  expresses  himself:  *  II 
any  one  should  take  upon  him  to  prove,  from  a  well- 
connected  comparison  of  phenomena,  that  thunder  is, 
in  the  hands  of  Nature,  what  electricity  is  in  ours,  and 
that  the  wonders  which  we  now  exhibit  at  our  pleasure 
are  little  imitations  of  those  great  effects  which  frighten 
us ;  I  avow  that  this  idea,  if  it  was  well  supported, 
would  give  me  a  great  deal  of  pleasure.'  He  then 
points  out  the  analogies  between  both,  and  continues 
thus  :  '  All  those  points  of  analogy,  which  I  have  been 
some  time  meditating,  begin  to  make  me  believe  that 
one  might,  by  taking  electricity  as  the  model,  form  to 
one's  self,  in  relation  to  thunder  and  lightning,  more 
perfect  and  more  probable  ideas  than  what  have  been 
offered  hitherto.' l 

These  views  were  prevalent  at  the  time  now  referred 
to,  and  out  of  them  grew  the  experimental  proof  by  the 
great  physical  philosopher,  Franklin,  of  the  substantial 
identity  of  the  lightning  flash  and  the  electric  spark. 

Franklin  was  twice  struck  senseless  by  the  electric 
shock.  He  afterwards  sent  the  discharge  of  two  large 
jars  through  six  robust  men  ;  they  fell  to  the  ground 
and  got  up  again  without  knowing  what  had  happened  ; 
they  neither  heard  nor  felt  the  discharge.  Priestley, 
who  made  many  valuable  contributions  to  electricity, 
received  the  charge  of  two  jars,  but  did  not  find  it 
painful. 

This  experience  agrees  with  mine.  Some  time  ago 
I  stood  in  this  room  with  a  charged  battery,  of  fifteen 
large  Ley  den  jars  be«ide  me.  Through  some  awkward- 
ness on  my  part  I  touched  the  wire  leading  from  the 
battery,  and  the  discharge  went  through  me.  For  a 
1  Priestley's  History  of  Electricity,'  pp.  51-52. 


Atmospheric  Electricity.  99 

lensible  interval  life  was  absolutely  blotted  out,  but 
there  was  no  trace  of  pain.  After  a  little  time  con- 
sciousness returned  ;  I  saw  confusedly  both  the  audience 
and  the  apparatus,  and  concluded  from  this,  and  from 
my  own  condition,  that  I  had  received  the  discharge. 
To  prevent  the  audience  from  being  alarmed,  I  made 
the  remark  that  it  had  often  been  my  desire  to  receive 
such  a  shock  accidentally,  and  that  my  wish  had  at 
length  been  fulfilled.  But  though  the  intellectual 
consciousness  of  my  position  returned  with  exceeding 
rapidity,  it  was  not  so  with  the  optical  consciousness. 
For,  while  making  the  foregoing  remark,  my  body  pre- 
sented to  my  eyes  the  appearance  of  a  number  of 
separate  pieces.  The  arms,  for  example,  were  detached 
from  the  trunk  and  suspended  in  the  air.  In  fact, 
memory,  and  the  power  of  reasoning,  appeared  to  be 
complete,  long  before  the  restoration  of  the  optic  nerve 
to  healthy  action. 

This  may  be  regarded  as  an  experimental  proof  that 
people  killed  by  lightning  suffer  no  pain. 

§  30.  Atmospheric  Electricity. 

The  air  at  all  times  can  be  proved  to  be  a  reservoir 
of  electricity,  which  undergoes  periodic  variation.  We 
have  seen  that  ingenious  men  began  soon  to  suspect 
a  common  origin  for  the  crackling  and  light  of  the 
electric  spark,  and  thunder  and  lightning.  The  greatest 
investigator  in  this  field  is  the  celebrated  Dr.  Franklin. 
He  made  an  exhaustive  comparison  of  the  effects  of 
electricity  and  those  of  lightning.  The  lightning  flash 
he  saw  was  of  the  same  shape  as  the  elongated  electric 
spark  ;  like  electricity,  lightning  strikes  pointed  objects 


KIIJ  Lessons  in  tiiectncity. 

in  preference  to  others :  lightning  pursues  the  path  of 
least  resistance  :  it  burns,  dissolves  metals,  rends  bodies 
asunder,  and  strikes  men  blind.  Franklin  imitated  all 
these  effects,  striking  a  pigeon  blind,  and  killing  a  hen 

FIQ.  54. 


and  turkey  by  the  electrical  discharge.  I  place  before 
y*m  in  fig.  54,  with  a  view  to  its  comparison  with  a  dis- 
charge of  forked  lightning,  the  long  spark  obtained 
from  an  effective  ebonite  machine,  furnished  with  a 
conductor  of  a  special  construction,  which  favours 
length  of  spark. 

Having  completely  satisfied  his  mind  by  this  com- 
parison of  the  identity  of  both  agents,  Franklin  pro- 
posed to  draw  electricity  from  the  clouds  by  a  pointed 
rod  erected  on  a  high  tower.  But  before  the  tower 
could  be  built  he  succeeded  in  his  object  by  means  of 
a  kite  with  a  pointed  wire  attached  to  it.  The  elec- 
tricity descended  by  the  hempen  string  which  held 
the  kite,  to  a  key  at  the  end  of  it,  the  key  being  sepa- 
rated from  the  observer  by  a  silken  string  held  in 
the  hand.  Franklin  thus  obtained  sparks,  and  charged 
a  Leyden  phial  with  atmospheric  electricity. 

But,  spurred  by  Franklin's  researches,  an  observer 
in  France  had  previously  proved  the  electrical  character 
of  lightning.  A  translation  of  Franklin's  writings  on 


Atmospheric  Electricity.  101 

t.'ie  subject  fell  into  the  hands  of  the  naturalist  Buffon, 
who  requested  his  friend  D'Alibard  to  revise  the  trans- 
lation. D'Alibard  was  thus  induced  to  erect  an  iron 
rod  40  feet  long,  supported  by  silk  strings,  and  ending 
in  a  sentry-box.  It  was  watched  by  an  old  dragoon 
nanitd  Coiffier,  who  on  the  10th  of  May,  1752,  heard  a 
clap  of  thunder,  and  immediately  afterwards  drew  sparks 
from  the  end  of  the  iron  rod. 

The  danger  of  experiments  with  metal  rods  was 
soon  illustrated.  Professor  Eichmann  of  St.  Peters- 
burg had  a  rod  raised  three  or  four  feet  above  the 
tiles  of  his  house.  It  was  connected  by  a  chain  with 
another  rod  in  his  room  ;  the  latter  rod  resting  in  a  glass 
vessel,  and  being  therefore  insulated  from  the  earth. 
On  the  6th  of  August,  1753,  a  thunder  cloud  discharged 
itself  agaiuut  the  external  rod ;  the  electricity  passed 
downwards  along  the  chain  ;  on  reaching  the  rod  below, 
it  was  stopped  by  the  glass  vessel,  darted  to  Richmann's 
head,  which  »vas  about  a  foot  distant,  and  killed  him 
on  the  spot.  Had  a  perfect  communication  existed 
between  the  lower  rod  and  the  earth,  the  lightning  in 
this  case  would  have  expended  itself  harmlessly. 

In  1749  Franklin  proposed  lightning  conductors. 
He  repeated  his  recommendation  in  1753.  He  was  op- 
posed on  two  grounds.  The  Abbe  Nollet,  and  those  who 
thought  with  him,  considered  it  as  impious  to  ward  off 
heaven's  lightnings,  as  for  a  child  to  ward  off  the 
chastening  rod  of  its  father.  Others  thought  that 
the  conductors  would  'invite 'the  lightning  to  break 
upon  them.  A  long  discussion  was  also  carried  on  as  to 
whether  the  conductors  should  be  blunt  or  pointed. 
Wilson  advocated  blunt  conductors  against  Franklin, 
Cavendish,  and  Watson.  He  so  influenced  George  III., 


102  Lessons  in  Electricity. 

hinting  that  the  points  were  a  republican  device  to 
injui'e  his  Majesty,  that  the  pointed  conductors  on 
Buckingham  House  were  changed  for  others  ending 
in  balls.  Experience  of  the  most  varied  kind  has 
justified  the  employment  of  pointed  conductors.  In 
1769  St.  Paul's  Cathedral  was  first  protected. 

The  most  decisive  evidence  in  favour  of  conductors 
was  obtained  from  ships ;  and  such  evidence  was 
needed,  to  overcome  the  obstinate  prejudice  of  seamen. 
Case  after  case  occurred  in  which  ships  unprotected  by 
conductors  were  singled  out  from  protected  ships,  and 
shattered  or  destroyed  by  lightning.  The  conductors 
were  at  first  made  movable,  being  hoisted  on  the  ap- 
proach of  a  thunderstorm ;  but  these  were  finally  aban- 
doned for  the  fixed  lightning  conductors  devised  by  the 
late  Sir  Snow  Harris.  The  saving  of  property  and 
life  by  this  obvious  outgrowth  of  electrical  research  is 
incalculable. 


§  31.  The  Returning  Stroke. 

In  the  year  1779  Charles,  Viscount  Mahon,  after- 
wards Earl  Stanhope,  published  his  *  Principles  of 
Electricity.'  On  the  title-page  of  the  book  stands  the 
following  remark  : — *  This  treatise  comprehends  an 
explanation  of  an  electrical  returning  stroke,  by  which 
fatal  effects  may  be  produced  even  at  a  vast  distance 
from  the  place  where  the  lightning  falls.' 

Lord  Mahon's  experiments,  which  are  models  of 
scientific  clearness  and  precision,  will  be  readily  under- 
stood by  reference  to  the  principles  of  electric  induction, 
with  which  you  are  now  so  familiar.  It  need  only  be 
noted  here  that  whenever  he  speaks  of  a  body  being 


The  Returning  Stroke. 


103 


plunged  in  an  '  electrical  atmosphere,'  he  means  that 
the  body  is  exposed  to  the  inductive  action  of  a  second 
electrified  body,  which  latter  he  supposed  to  be  sur- 
rounded by  such  an  atmosphere. 

A  few  extracts  from  his  work  will  give  a  clear  notion 
of  the  nature  of  his  discovery  : — 

'I  placed  an  insulated  metallic  cylinder,  AB,  fig.  55, 

Fio.  55. 


within  the  electrical  atmosphere  of  the  prime  con- 
ductor [P  c]  when  charged,  but  beyond  the  striking  dis- 
tance. The  distance  between  the  near  end  A  of  the 
insulated  metallic  body  and  the  side  of  the  prime  con- 
ductor was  20  inches.  The  body  A  B  was  of  brass,  of  a 
cylindrical  form,  1 8  inches  long  by  2  inches  in  diameter. 
I  then  placed  another  insulated  brass  body  EF,  40  inches 


104  Lessons  in  Electricity. 

long  by  about  3|  inches  in  diameter,  with  its  end  B  at 
the  distance  of  about  one-tenth  of  an  inch  from  the  end  B 
of  the  other  metallic  body  AB.  I  electrified  the  prime 
conductor.  All  the  time  that  it  was  receiving  its  plus 
charge  of  electricity  there  passed  a  great  number  of 
weak  (red  or  purple)  sparks  from  the  end  B  of  the  near 
body  A  B  into  the  end  E  of  the  remote  body  E  r.' 

Make  clear  to  your  mind  the  origin  of  this  stream 
of  weak  red  or  purple  sparks.  It  is  obviously  due  to 
the  inductive  action  of  the  prime  conductor  p  c  upon 
the  body  AB.  The  positive  electricity  of  AB  being 
repelled  by  the  prime  conductor,  passed  as  a  stream  of 
sparks  to  E  F. 

'  When  the  prime  conductor,  having  received  its 
full  charge,  came  suddenly  to  discharge,  with  an  ex- 
plosion, its  superabundant  electricity  on  a  large  brass 
ball  L,  which  was  made  to  communicate  with  the  earth, 
it  always  happened  that  the  electrical  fluid,  which  had 
been  gradually  expelled  from  the  body  A  B  and  driven 
into  the  body  E  F,  did  suddenly  return  from  the  body  E  F 
into  the  body  A  B,  in  a  strong  and  bright  spark,  at  the  very 
instant  that  the  explosion  took  place  upon  the  ball  L. 
'  This  I  call  the  electrical  returning  stroke? 
For  the  two  conductors  Lord  Mahon  then  sub- 
stituted his  own  body  and  that  of  another  person,  both 
of  them  standing  upon  insulating  stools.  He  continues 
thus  :— 

'  I  placed  myself  upon  an  insulating  stool  E  (fig.  56), 
so  as  to  have  my  right  arm  A  at  the  distance  of  about  20 
inches  from  a  large  prime  conductor ;  another  person, 
standing  upon  another  insulating  stool  K,  brought  his 
right  hand  F  within  one-quarter  of  an  inch  of  my  left 
hand  B. 


The  Returning  Stroke. 


105 


4  When  the  prime  conductor  began  to  receive  ita 
plus  charge  of  electricity,  we  felt  the  electrical  fluid 
running  out  of  my  hand  B  into  his  hand  F. 


FIG.  56. 


'  When  we  separated  our  hands  B  and  F  a  little,  the 
electricity  passed  between  us  in  small  sparks,  which 
sparks  increased  in  sharpness  the  farther  we  removed 
our  hands  B  and  F  asunder,  until  we  had  brought  them 
quite  out  of  a  striking  distance.  The  intervals  of  time 
between  these  departing  sparks  increased  also  the 
more  the  distance  between  our  hands  B  and  F  was  in- 
creased, as  must  necessarily  be  the  case. 

'  As  soon  as  the  prime  conductor  came  suddenly  to 
discharge  its  electricity  upon  the  ball  L,  the  super- 
abundant electricity  which  the  other  person  had  re- 
ceived from  my  body  did  then  return  from  him  to  me 
in  a  sharp  spark,  which  issued  from  his  hand  F  at  the 
very  instant  that  the  explosion  of  the  piime  conductoi 
took  place  upon  the  ball  L. 


106  Lessons  in  Electricity. 

'  I  still  continued  upon  the  insulating  stool  E,  and  1 
desired  the  other  person  to  stand  upon  the  floor.  The 
returning  stroke  between  us  was  still  stronger  than  it 
had  yet  been.  The  reason  of  it  was  this : — the  other 
person  being  no  longer  insulated,  transmitted  his  super- 
abundant electricity  freely  into  the  earth.  I  conse- 
quently became  still  more  negative  than  before. 

'  Now,  when  the  returning  stroke  came  to  take 
place,  not  only  the  electricity  which  had  passed  from 
my  body  into  the  body  of  the  other  person,  but  also  the 
electricity  which  had  passed  from  my  body  into  the 
earth  (through  the  other  person),  did  suddenly  return 
upon  me  from  his  hand  F  to  my  hand  B,  at  the  same 
instant  that  the  discharge  of  the  prime  conductor  took 
place  upon  the  ball  L.  This  caused  the  returning  stroke 
to  be  stronger  than  before.' 

Lord  Mahon  fused  metals,  and  produced  strong 
physiological  effects  by  the  return  stroke. 

In  nature  disastrous  effects  may  be  produced  by  the 
return  stroke.  The  earth's  surface,  and  animals  or  men 
upon  it,  maybe  powerfully  influenced  by  one  end  of  an 
electrified  cloud.  Discharge  may  occur  at  the  other 
end,  possibly  miles  away.  The  restoration  of  the  elec- 
tric equilibrium  by  the  return  shock  may  be  so  violent 
as  to  cause  death. 

This  was  clearly  seen  and  illustrated  by  Lord  Mahon. 
Fig.  57  is  a  reduced  copy  of  his  illustration.  A  B  c  is 
the  electrified  cloud,  the  two  ends  of  which,  A  and  c, 
come  near  the  earth.  The  discharge  occurs  at  c.  A 
man  at  F  is  killed  by  the  returning  stroke,  while  the 
people  at  D,  nearer  to  the  place  of  discharge,  but  farther 
from  the  cloud,  are  uninjured. 

With  the  view  of  still  further  testing  your  know- 


The  Returning  Stroke.  107 

ledge  of  induction,  I  have  here  copied  a  portion  of  this 
admirable  essay ;  but  the  entire  memoir  of  Lord  Mahon 

FIG.  57. 


would  constitute  a  most  useful  and  interesting  lesson 
in  electricity. 

For  our  own  instruction  we  can  illustrate  the  return 
shock  thus : — Connect  one  arm  of  your  universal  dis- 
charger, fig.  49,  with  a  conductor  like  c,  fig.  20,  and 
the  other  arm  with  the  earth.  Bring  c  within  a  few 
inches  of  your  prime  conductor,  but  not  within  striking 
distance ;  on  working  the  machine  a  stream  of  feeble 
sparks  will  pass  from  point  to  point  of  the  discharger. 
Let  the  prime  conductor  be  discharged  from  time  to 
time  by  an  assistant ;  at  every  discharge  the  returning 
stroke  is  announced  by  a  flash  between  the  points  of 
the  discharger  at  8,  If  gun-cotton  with  a  little 
fulminating  powder  scattered  on  it,  or  a  fine  silver  wire, 
be  introduced  between  the  points  of  the  discharger,  the 
one  is  exploded  and  the  other  deflagrated. 


108  Lessons  in  Electricity. 

The  stream  of  repelled  sparks  first  seen  may  be  en- 
tirely abolished  by  establishing  an  imperfect  connexion 
between  the  conductor  c  and  the  earth:  a  chain  resting 
upon  the  dry  table  on  which  the  conductor  stands 
will  do.  The  chain  permits  the  feebler  sparks  to  pass 
through  it  in  preference  to  crossing  the  space  s ;  but 
the  returning  stroke  is  too  strong  and  sudden  to  find 
a  sufficiently  open  channel  through  the  table  and  chain, 
and  on  the  discharge  of  the  prime  conductor  the  spark 
is  seen. 

It  was  the  action  of  the  return  shock  upon  a  dead 
frog's  limbs,  observed  in  the  laboratory  of  Professor 
Galvani,  that  led  to  Galvani's  experiments  on  animal 
electricity  ;  and  led  further  to  the  discovery,  by  Volta, 
of  the  electricity  which  bears  his  name. 

§  32.  The  Leyden  Battery,  its  Currents,  and  some 
of  their  Effects. 

In  the  ordinary  Leyden  battery  described  in  §  19  all 
the  inner  coatings  are  connected  together,  and  all  the 
outer  coatings  are  also  connected  together.  Such  a 
battery  acts  as  a  single  large  jar  of  extraordinary  di- 
mensions. 

Wires  are  warmed  by  a  moderate  electric  discharge ; 
by  augmenting  the  charge  they  are  caused  to  glow  ; 
with  a  strengthened  charge  the  metal  is  torn  to  pieces ; 
fusion  follows ;  and  by  still  stronger  charges  the  wires 
are  reduced  to  metallic  dust  and  vapour. 

For  such  experiments  the  wire  must  be  thin. 
Without  resistance  we  can  have  no  heat,  and  when  the 
wire  is  thick  we  have  little  resistance.  The  mechanism 
of  the  discharge,  as  shown  by  the  figures  produced,  is 


The  Ley  den  Battery.  109 

different  in  different  wires.  The  figure  produced  by 
the  dust  of  a  deflagrated  silver  wire  on  white  paper  is 
shown  in  fig.  58. 

FJO.  58. 


When  the  discharge  of  a  powerful  battery  is  sent 
through  a  long  steel  chain  with  the  ends  of  its  links 
unsoldered,  the  sparks  between  the  unsoldered  links 
carry  the  incandescent  particles  of  the  steel  along  with 
them.  These  are  consumed  in  the  air,  a  momentary 
blaze  occurring  along  the  entire  chain.  Chain  cables 
have  been  fused  by  being  made  the  channels  of  a  flash 
of  lightning. 

Eetaining  our  conception  of  an  electric  fluid,  at 
this  point  we  naturally  add  to  it  the  conception  of  a 
current.  It  is  the  electric  current  which  produces  the 
effects  just  described.  In  many  of  our  former  experi- 
ments we  had  electricity  at  rest  (static  electricity), 
here  we  have  electricity  in  motion  (dynamic  elec- 
tricity). 

Sending  the  current  from  a  battery  through  a  flat 
spiral  (the  primary)  formed  of  fifty  or  sixty  feet  of 
copper  wire,  and  placing  within  a  little  distance  of 
it  a  second  similar  spiral  (the  secondary)  with  its 
ends  connected ;  the  passage  of  the  current  in  the  first 
spiral  excites  in  the  second  a  current,  which  is  competent 
to  deflagrate  wires,  and  to  produce  all  the  other  effects 
of  the  electrical  discharge.  Even  when  the  spirals  are 


110  Lessons  in  Electricity. 

some  feet  asunder,  the  shock  produced  by  the  secondary 
current  is  still  manifest. 

The  current  from  the  secondary  spiral  may  be 
carried  round  a  third ;  and  this  third  spiral  may  be 
allowed  to  act  upon  a  fourth,  exactly  as  the  primary  did 
upon  the  secondary.  A  tertiary  current  is  thus  evoked 
by  the  secondary  in  the  fourth  spiral. 

Carrying  this  tertiary  current  round  a  fifth  spiral, 
and  causing  it  to  act  inductively  upon  a  sixth,  we  obtain 
in  the  latter  a  current  of  the  fourth  order.  In  this  way 
we  generate  a  long  progeny  of  currents,  all  of  them 
having  the  current  sent  from  the  battery  through  the 
first  spiral,  for  a  common  progenitor.  To  Prof.  Henry 
of  the  United  States,  and  to  Prof.  Eiess  of  Berlin,  we 
are  indebted  for  the  investigation  of  the  laws  of  these 
currents.  These  researches,  however,  were  subsequent 
to,  and  were  indeed  suggested  by,  experiments  of  a 
similar  character  previously  made  by  Faraday  with 
Voltaic  electricity. 

Besides  the  electricity  of  friction  and  induction  we 
have  the  following  sources  and  forms  of  this  power. 

The  contact  of  dissimilar  metals  produces  electricity. 

The  contact  of  metals  with  liquids  produces  elec- 
tricity. 

A  mere  variation  of  the  character  of  the  contact  of 
two  bodies  produces  electricity. 

Chemical  action  produces  a  continuous  flow  of  elec- 
tricity (Voltaic  electricity). 

Heat,  suitably  applied  to  dissimilar  metals,  produces 
a  continuous  flow  of  electricity  (thermo-electricity). 

The  heating  and  cooling  of  certain  crystals  produce 
electricity  (pyro-electricity). 


Conclusion.  1 1 1 

The  motion  of  magnets,  and  of  bodies  carrying  elec- 
tric currents,  produces  electricity  (magneto-electricity). 

The  friction  of  sand  against  a  metal  plate  produces 
electricity. 

The  friction  of  condensed  water-particles  against  a 
safety  valve,  or  better  still  against  a  box-wood  nozzle 
through  which  steam  is  driven,  produces  electricity 
(Armstrong's  hydro-electric  machine). 

These  are  different  manifestations  of  one  and  the 
same  power ;  and  they  are  all  evoked  by  an  equivalent 
expenditure  of  some  other  power. 

Conclusion. 

Our  experimental  researches  end  here,  I  would 
now  bespeak  your  attention  for  five  minutes  longer. 
The  expensiveness  of  apparatus  is  sometimes  urged  as 
an  obstacle  to  the  introduction  of  science  into  schools. 
I  hope  it  has  been  shown  that  the  obstacle  is  not  a  real 
one.  Leaving  out  of  account  the  few  larger  experi- 
ments, which  have  contributed  but  little  to  our  know- 
ledge, it  is  manifest  that  the  wise  expenditure  of  a  couple 
of  guineas  would  enable  any  competent  teacher  to  place 
the  leading  facts  and  principles  of  factional  electricity 
completely  at  the  command  of  his  pupils;  giving  them 
thereby  precious  knowledge,  and  still  more  precious  in- 
tellectual discipline — a  discipline  which  invokes  obser- 
vation, reflection,  prevision  by  the  exercise  of  reason,  and 
experimental  verification. 

And  here,  if  I  might  venture  to  do  so,  I  would  urge 
upon  the  science  teachers  of  our  public  and  other  schools 
that  the  immediate  future  of  science  as  a  factor  in  English 
education  depends  mainly  upon  them.  I  would  respect- 


112  Lessons  in  Electricity. 

fully  submit  to  them  whether  it  would  not  be  a  mistake 
to  direct  their  attention  at  present  to  the  collection  of 
costly  apparatus.  Their  principal  function  just  now  is 
to  arouse  a  love  for  scientific  study.  This  is  best  done  by 
the  exhibition  of  the  needful  facts  and  principles  with 
the  simplest  possible  appliances,  and  by  bringing  their 
pupils  into  contact  with  actual  experimental  work. 

The  very  time  and  thought  spent  in  devising  such 
simple  instruments  will  give  the  teacher  himself  a 
grasp  and  mastery  of  his  subject  which  he  could  not 
otherwise  obtain;  but  it  ought  to  be  known  by  the  head 
masters  of  our  schools  that  time  is  needed,  not  only  for 
devising  such  instruments,  but  also  for  preparing  the 
experiments  to  be  made  with  them  after  they  have  been 
devised.  No  science  teacher  is  "fit  to  meet  his  class 
without  this  distinct  and  special  preparation  before 
every  lesson.  His  experiments  are  part  and  parcel  of 
his  language,  and  they  ought  to  be  as  strict  in  logic, 
and  as  free  from  stammering,  as  his  spoken  words.  To 
make  them  so  may  imply  an  expenditure  of  time  which 
few  head  masters  now  contemplate,  but  it  is  a  necessary 
expenditure,  and  they  will  act  wisely  in  making  pro- 
vision for  it. 

To  them,  moreover,  in  words  of  friendly  warning,  I 
would  say,  make  room  for  science  by  your  own  healthy 
and  spontaneous  action,  and  do  not  wait  until  it  is 
forced  upon  you  by  revolutionary  pressure  from  without. 
The  condition  of  things  now  existing  cannot  continue. 
Its  simple  statement  suffices  to  call  down  upon  it  the 
condemnation  of  every  thoughtful  mind.  With  refer- 
ence to  the  report  of  a  Commission  appointed  last  year 
to  enquire  into  the  scientific  instruction  of  this  country, 
Sir  John  Lubbock  writes  as  follows  : — '  The  Commis- 


Conclusion.  113 

sioners  have  published  returns  from  more  than  a  hundred 
and  twenty  of  the  larger  endowed  schools.  In  more 
than  half  of  these  no  science  whatever  is  taught ;  only 
thirteen  have  a  laboratory,  and  only  eighteen  possess 
any  scientific  apparatus.  Out  of  the  whole  number,  less 
than  twenty  schools  devote  as  much  as  four  hours  a 
week  to  science,  and  only  thirteen  attach  any  weight  at 
all  to  scientific  subjects  in  the  examinations.' 

Well  may  the  Commissioners  pronounce  such  a 
state  of  things  to  be  nothing  less  than  a  national 
calamity  !  If  persisted  in,  it  will  assuredly  be  followed 
by  a  reaction  which  the  truest  friends  of  classical 
culture  in  England  will  have  the  greatest  reason  to 
deplore. 


A  List  of  Apparatus 

NECESSARY  TO  ILLUSTRATE  THIS  BOOK. 


One  latest  Improved  Holtz  Electrical  Induc- 
tion Machine  with  12  inch  revolving  plate, 
and  giving  from  5^  to  6  inch  spark,  com- 
plete, with  catskin  and  piece  of  hard  rubber,  $25  00 

One  quart  Leyden  jar        .  .             .             .  1  50 

One  electrical  flier        .  .             .                      1  25 

One  gold-leaf  electroscope  .             .             .  4  50 

One  pith-ball  electrometer  .             .                       1  00 

One  box  pith           .             .  .             .             .25 

One  electrical  bucket   .  .             .                      1  00 

One  box  lycopodium           .  .             .             .50 

One  insulated  brass  plate  .             .                      1  00 

One  insulated  stool             .  V           .             .  3  00 

One  illuminated  egg  stand  .             .                      2  00 

One  electrical  umbrella      .  .             .             .  1  25 

One  electrical  discharger  .             .                      2  50 

One  Leyden  jar  with  movable  coatings  .  .  3  00 


ii  Price-List  of  Apparatus. 

One  ether-cup    .             .             .             .  $1  25 
Three  Geissler's  or  vacuum  tubes  (assorted  colors)  3  00 

One  lightning-tube  two  feet  long      .  .         3  50 

One  box  brass  chain           .             .             .  .25 

One  gunpowder-cup      .             .             .  1  25 

One  head  of  hair    .  .  1  25 


$68  25 

Complete  as  above,  carefully  packed  and  boxed 

for  shipment    ....  $66  00 


J.  &  H.  BEI\GE, 

Manufacturers  of  Electrical  and  Philosophical  Appara- 
tus of  best  quality  and  latest  improved  designs, 

No.  191  Greenwich  St.,  N.  Y. 

Descriptive  and   Illustrated   Circulars  sent  free. 


N.  B. — This  collection  of  apparatus  is  all-sufficient 
to  fully  illustrate  all  of  the  experiments  mentioned  in 
this  book. 


l^fT0  Every  piece  of  apparatus  is  constructed  in 
the  best  manner  and  of  best  materials,  and  carefully 
tested  before  sending  from  our  shops. 

J.  &  H.  BERGE. 


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"  The  Collective  Wisdom." 
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The  Americans.! 
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"  It  was  inevitable  that  his  essays  should  be  called  for  in  a  completed  form,  and  they 
will  be  a  source  of  delight  and  profit  to  all  who  read  them.  He  has  always  commanded 
a  hearing,  and  as  a  master  of  the  literary  style  in  writing  scientific  essays  he  is  worthy 
of  a  place  among  the  great  English  essayists  of  the  day.  This  edition  of  his  essays 
will  be  widely  read,  and  gives  his  scientific '"work  a  permanent  form." — Boston  Herald. 

"A  man  whose  brilliancy  is  so  constant  as  that  of  Prof.  Huxley  will  always  com- 
mand readers;  and  the  utterances  which  are  here  collected  are  not  the  least  in  weight 
and  luminous  beauty  of  those  with  which  the  author  has  long  delighted  the  reading 
world."— Philadelphia  Press. 

"The  connected  arrangement  of  the  essays  which  their  reissue  permits  brings  into 
fuller  relief  Mr.  Huxley's  masterly  powers  of  exposition.  Sweeping  the  subject-matter 
clear  of  all  logomachies,  he  lets  the  light  of  common  day  fall  upon  it.  He  shows  that 
the  place  of  hypothesis  in  science,  as  the  starting  point  of  verification  of  the  phenomena 
to  be  explained,  is  but  an  extension  of  the  assumptions  which  underlie  actio-s  in  eveiy- 
day  affairs;  and  that  the  method  of  scientific  investigation  is  only  the  method  which 
rules  the  ordinary  business  of  life." — London  Chronicle. 


New  York:   D.  APPLETON    fc  CO.,  72 


001  023 


I 


STATE  NORMALSCHUOL. 


